Whilst it is possible it could be an FP if it can be guaranteed it hasn't been changed, but new and updates signatures are added and as such may pick up something not previously detected (especially if it is a dormant file).
No matter what if avast considers it infected it will let you Restore it and will promptly alert when it lands in the original location and guess where it will end up, yes right back in the chest.
So it has to be investigated:
You could also check the offending/suspect file at:
VirusTotal - Multi engine on-line virus scanner and
report the findings here the URL in the Address bar of the VT results page. You can't do this with the file securely in the chest, you need to extract it to a temporary (not original) location first, see below.
- avast5 - Create a folder called Suspect in the C:\ drive. Now exclude that folder in the
File System Shield, Expert Settings, Exclusions, Add, type (or copy and paste) C:\Suspect\* That will stop the File System Shield scanning any file you put in that folder. Now enter the chest again and Extract the file to the Suspect folder and upload it to VT.