Three wheels on my ’00 have given me problems with mystery leaks. In my case it was leaking at the bead seal location on the rim. When you take the tire off the rim, you can see where the aluminum starts to corrode under the chrome plating causing the chrome to crack and flake. Usually when this happens, the leak is not there all the time and will not show up if you dunk it in water. The car has to be sitting on the wheel in a certain position for it to leak.

To test for this problem, apply a soap solution to inside and outside of rims with the wheel on the car.
Check all around beads for bubbles
Back the car up a couple inches and check for leaks again.
Repeat until wheel has made a full rotation.
If you don’t see anything then your problem is elsewhere.

Most of the time the leak was on the outer bead. You can also check the tread in the same manner if you think you missed a nail or something.

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For fixing this, you have to be thorough with removing all loose corrosion and painting the bad surfaces. I did the first 2 myself and they are holding up fine. The others I had the tire store do but one started leaking again after about 18 months.

I have chrome-plated wheels over aluminum and I suspect a seep into the chrome plating on top of the rim between the beads on one. I read that the chrome goes on there but the wheel isn't prepped so the chrome doesn't stick as well. Don't know if that'* true or not.

As for finding slow seeps I've used puddles in a parking lot and dish detergent to coat a tire and look for leaks when the tread is in the low end with the weight of the car flexing the rubber.

Seeps at the bead where aluminum has oxidized probably seep only as they flex when the weight moves onto them and then off. But they don't seep while stationary.

I would have thought the same years ago before actually seeing it, but mine did leak when stationary with the wheel only in a certain postion - and it was repeatable.

I found this to be my problem after being in the same place as malatu:

That'* exactly what it was. The wheel was corroded at the edge of the bead. The tire folks said they were able to grind/buff the corrosion. I suspect this will be a temporary fix.

....."Wheel" see if this works!