If the light was yellow, and you found only 2 qts low you should be fine.. dont worry about it any further. the warning lights did there job.
If they got the filter kit from BMW, then the prob replaced the oring on the filter cap with the new one supplied. Odds are they didnt lube it and it got caught on the tight plastic housing threads and squished up..
in all honesty they should have noticed as they were resecuring the cap.
dont feel bad... have seen and heard it happen dozens of times..
