I've seen this happen due to colder temps, but I've ALSO seen something more alarming happen on 4 out of 4 big single cars (factory ECU) that I've been close to.
Here's what happened on ALL 4 cars. Keep in mind that these 4 cars have 4 different turbos, two different headers, and 3 different fuel systems, 3 different fuel controllers, and VPC temp sensors located in various places on the intake tract. The common thread is that they are all on VPC, fuel controller, and factory ECU with raised rev limiters.
The cars were carefully tuned to about 11.5:1 (under boost) on a wideband. For the rest of the day, and perhaps for a few days afterward, everything stayed the same -- AFR in the 11.5 range. Then, a few days later with NO ADJUSTMENTS whatsoever on any of the electronics, and NO WEATHER CHANGE, the cars went anywhere from about 1 full point to 2+ full points LEANER. That's 13.5:1, up from 11.5:1. The AFR was then re-tuned and the cars held that tune indefinitely, perhaps with minor tweaking as is often needed to keep AFRs within 2 or 3 tenths of target from day to day or week to week.
So, for anyone who thinks they can go to the dyno or borow a wideband, tune the AFR, then just forget it, they better think again.
More detailed info on this can be found in the FAQ of www.widebando2.com