I have an SUV with factory 16" steel wheels and P235/70R16 tires. The tires have just under 30,000 miles of gentle driving on them. Two of the tires have been leaking quite a bit (about 10 psi per week). I brought the vehicle to two different reputable independent mechanics, and they took the tires off, and couldn't find a problem. They applied bead sealant (which I know is controversial). Both also replaced the valve stems as a guess after they couldn't find leaks in the tires anywhere. I also checked for leaks myself with soapy water in a spray bottle.
If the professionals couldn't find leaks anywhere, what should I do next? Should I bring it to additional shops until someone can find the problem? Are the mechanics looking in the wrong place? Could the wheels be the issue? They seem to be in decent shape from what I can see.