Starting to sound potentially like a bug in AVAST if other things (Mbam) are not picking up anything.
Given the nature of temp files, it may well be that the file is getting modified as it's being scanned and AVAST isn't coping well with that.
Never say never as they say, but it seems like you've (correctly) gone to the effort of scanning with other software and found nothing it's difficult to say if it's something to worry about.