You do realize it's an antivirus that scans things on-access? It doesn't have to be you executing or accessing the files. It can be Windows search, prefetch, Steam client doing updates, numerous things.
I don't have Steam or anything else installed that should be scanning my drive. Windows search shouldn't be doing anything on its own.
avast! just intercepts access events and scans said files. There is nothing evil behind it and every antivirus will behave the same minus certain differences because they don't scan same scope of files and in exact same way.
If Avast is just intercepting some other process scanning my drive, why doesn't that process show up in Process Monitor alongside Avast?
You missed the point of Rej's clarification. Any time you open a file (images, exes, doc(x), ppt(x), etc) Avast! will scan it do make sure it's not doing anything malicious. Evidently, you don't know why Avast! would chose to scan documents... so let me point you in the right direction. Emotet, quite possibly the most prevalent piece of malware is spread using an exploitation in word documents. It's highly effective, extremely dangerous, and unfortunately for most users, they'd never think "Oh, that PNG or DOCX file could contain malware!".
https://blog.malwarebytes.com/detections/trojan-emotet/The United Nations (yes, the UN) was recently hit with a cyber attack using none other then Emotet. Last confirmed report I had indicated 40+ core servers compromised in the attack.
https://www.forbes.com/sites/daveywinder/2020/01/30/united-nations-confirms-serious-cyberattack-with-42-core-servers-compromised/#ad03cb3633daThat is why Avast! is scanning documents/exe/images/dlls that are opened. This is perfectly normal behaviour for any antivirus. In fact, I'd say if it WASN'T doing it, I'd be suspicious.