Well, it was definitely the SSAO. I played with a few other graphics settings but it doesn't seem to be caused by interaction with any other effect, just SSAO.
The issue is instantly visible regardless of low/medium/high SSAO setting, and gone when SSAO is switched off.
Really weird. I guess since you've found the culprit, I don't have to go looking... Maybe I will take your earlier advice and enjoy my ignorant bliss