About an hour and twenty minutes ago I had a fullscreen application running, so didn’t see a popup or notice until just now, but Steam.exe was flagged and set up my Virus Chest. Having scanned it in the chest it said “no virus”, and I’m fairly sure it’s the legitimate Valve file, in the right place and all so I restored it and ran a scan and it all seems fine, but I just seemed very strange to have Steam flagged.
EDIT: And it appears I’m not the only one to have had this issue today: https://www.reddit.com/r/Steam/comments/3q0ugr/avast_identifying_steamexe_as_malware_when/