Ive had this issue with my car where CEL comes on without any errors on Rheingold. the tuner had to revert to another version of their tune where it would allow the codes to show.

I have read this on the e90 forums as well, it appears the tune routinely erases certain codes to keep the cel from coming on. Im not sure exactly how it works but you are left troubleshooting your car blindly.

Please do let us know what method the tuner uses to try to access the errors as i do not think there is any "back door"

I had my car take over 650 miles to show a code as well so how long have u driven it since it began spuddering?