Annoying SES light, search function didn't find my problem

[preppy]

So I have the dreaded SES light on my 01 4.4i. I've had it come and go for the past 6-7 months. It's only happened after I moved from Maryland to Denver, CO last August. Before being in CO, the x5 has been in TX and MD from 01-07.

So I take the x5 to a good indy shop to have the code read and to repair whatever the problem is (crossed my fingers it was a bum gas cap). Well, they read it and it tells them the car is running rich...okay...well, then they reset the SES to figure out what specifically is causing the car to run rich. Nothing odd, mechanically/eletrically shows up and the SES light doesn't come back on.

They run a battery of tests on her and still nothing, the only thing off on it is that the alternator at one point put out a really low voltage, but then corrected itself.

So we decided to give the x5 the weekend to see if the SES light comes back on. I was getting slightly poor gas mileage before I took it in (10-12 by rough guestimation), but after resetting that, the x5 actually gets about 13-15, which is about right for driving around town in traffic.

So what could be wrong with my x5? Should I just have them order me a new gas cap regardless?