avast! does not scan removable drives when they are attached - and never did.
It may be possible that some other program is "touching" the files... some old versions of avast! were not able to distinguish between "reading" and "starting" of executable files on some operating systems, so they scanned the files even though it wasn't necessary.
It might be your case - hard to say without knowing the filename, at least.
Anyway, you can configure Standard Shield provider to scan selected files "on open", if you wish.