This type of leak is caused by the bell-to-bow brace. You can look down inside the bell. If it's a newer horn with perfect lacquer, you can usually see a deformation in the reflection right at that point. If it's an older tarnished horn, you can feel the surface on the inside of the bell where the bell-to-bow brace connects and feel for any deformation in the tube. If you feel anything, then it is very likely that the other end of the brace, i.e., the attachment point at the body tube, has also suffered some deformation.
The deformation can be the result of a single incident or, just as likely, caused by daily bouncing in the case over time. If the horn has been carried in the common type of case where the horn is a little loose, small bumps over time can be a problem. A seemingly insignificant bump can cause the weight of the bell to push in on the body tube even when the horn is in the case. Of course if your sax stand falls over or you have some other accident, the same forces can apply and you will be very aware of an issue, but even still it may be hard to pin point the real damage.
This might be hard to see in a photo, but it is not too difficult to see in real life, especially if the horn is a newer shiny lacquered horn. I've never owned one of those, so this is an example of a 1950's Buescher Aristocrat that is a bit grungy. What we are looking for is a reflection that shows that the brace has pushed in the tube. A leak light down the body tube might also reveal a deformation, but again, the leak light isn't always the best test.
This area looks a little suspect. The light reflection just below the diamond-shaped foot of the bell-to-body brace appears to curve up slightly as it goes from left to right towards the diamond-shaped foot of the post on the right. That reflection should be straight (a good thing to know when looking at Ebay pictures). The post foot is strong and, from the look of the reflection, suffered no injury. What we are looking at is the hint that the bell brace has been pushed into the body tube a tiny bit. I need to get the light just right and check from other angles.
It is actually the other side of the bell brace foot diamond that has a more pronounced little dent, partially hidden by the pad cup and right next to the tone hole. The issue isn't so much the little dent, but the fact that a dent on the body tube is going to be right next to a tone hole. Let's look closer at the tone hole. You can see that the bell brace is right behind the tone hole chimney.
That looks okay and I couldn't see any problems with the leak light. Sometimes a little dent like this can have no effect on the tone hole and doesn't cause any playability issues.
You can see that when examining this area using a leak light and looking from the other side, my view would be blocked by the brace, the cup arm, and the rods. Plus, if a leak is at the top of the longest part of the tone hole chimney, that is where the traditional leak light is least effective.
Here's a simple way to check whether the tone hole is flat using a straight edge and leaving the key pad on. For a straight edge, I need something narrow and thin enough to easily insert over the tone hole, and thick enough not to flex. I don't need to cover the entire tone hole, as a generally would if I had the horn completely stripped down. I'm using a .025" feeler gauge. It is possible to flex it, but I laid it on my glass top bench to ensure that it is normally flat, and then I lay it on the tone hole. With the bench light behind the tone hole, I can examine the lip on the back side of the tone hole, right where I suspect there may be a problem.
Yikes! As I slide the feeler around the edges of the tone hole, I can see what the almost invisible brace dent has done to the closest tone hole. The foot of the bell brace corresponds to the apex of the deformation, so I'm certain that the tiny bow brace dent is the cause. The rest of the tone hole is still level, or sufficiently level to easily float the pad, but the back area is going to be a problem to fix now that we have detected the issue.
There are four methods to remedy this situation. The first was apparently already used. Put on new pads and hope that the pad forms a seat deep enough to make up for the tone hole being out of level. In this case, the pad formed a seat deep enough to hide the leak from the leak light, but that's not what is needed. We need it so that air can't pass through, not that light can't be seen.
If saxophone repair was my bread and butter, I would use the second method and tell the owner that this can be fixed by removing the lower stack, leveling the stack (probably by filing the tone hole chimney, which is not the best method), and floating a new pad. I would also quote my hourly rate or an estimate of the cost. Depending on where you live, this may cost upwards of $100.
Since it is my own horn and I'm not getting paid, I'll just refloat the existing pad (which is brand new from a recent repad). But, I now know that this tone hole isn't level and exactly where it isn't level. For the third method, I heat up the pad cup (since most pads are floated with shellac), lightly float the pad while the cup is cooling, and then with the shellac still in it's plastic state but losing it's viscosity, I use a pin to pull down only the back area of the pad (having first practiced a bit so that I know that I can get a hold of the pad with the pin when the time is just right). Having pulled down that back little area too far (so that it strikes the tone hole first, I again lightly close the pad to conform with the tone hole. What I have done is floated a pad so that the pad isn't flat because I know that the tone hole isn't flat. I call this a "taco" pad, although the pad isn't folded over nearly as much as a tortilla used to make a taco. In fact, I'll be the only one who knows that the pad isn't perfectly flat.
Now for the fourth method, the one I used here. Because this is a Buescher with snap-in pads, lifting part of the pad out while the shellac is molten won't work, as there is no shellac. Instead, I'm going to shim the pad. To do that, I need to know how thick my shim should be and where it should be placed. If you go back and look at the picture of the light shining through the low spot, you can "get a feel" for necessary thickness based on the feeler gauge that I used. I'm going to look for a piece of card stock that's just under .025" thick. Here it is (this is actually the card stock from the old reed holders that show up with vintage horns).
I cut a wedge shaped piece and stick it to the back of the Buescher snap-in pad. I put a discreet mark on the face of the pad so that I know exactly where my "high spot" is when I replace the pad and make sure that it corresponds with my known "low spot" on the tone hole. But before I replace it, I place the pad on my bench face up and press down around the shim. This deforms the aluminum backing used on the Buescher pads. I now have a taco pad that snaps into place, leaving the shim in just to make sure that it retains the taco shape.
Since it is my own horn, I can make sure that the horn plays perfect without it having to be perfect. I can then use the money that I would have paid to a tech to buy yet another mouthpiece! The only downside is that should this horn have a new owner, and the new owner visits a tech for a repad and that tech, like the prior tech, doesn't catch the tone hole issue, it will leak again and return to being one of those "stuffy" vintage horns. The issue will be hard to miss during a full repad because when the pad is removed there will be a shim on the back of the pad. That should give the tech a clue.
The vacuum method of leak detection is great for finding leaks, meaning that you will find whether the horn is leaking somewhere. It's not so great for actually locating the leaks. That's your job. The tone hole at the foot of the bell brace is one of the places to check first.