AMCAS Instruction Manual 2018, page 27:
You must answer Yes to this question if you were ever the recipient of any institutional
action by any college or medical school for unacceptable academic performance or
conduct violation, even if such action did not interrupt your enrollment or require you to
withdraw. You must answer Yes even if the action does not appear on or has been deleted
or expunged from your official transcripts due to institutional policy or personal petition.
If you report it as required and your undergrad institution responds to your request by saying that it has no record of IAs committed in 2011 (or whenever), then the medical school is likely to be satisfied with that response. The alternative would be for the medical school to assume that you are lying about having an IA when you don't actually have an IA and that seems completely crazy, doesn't it?