It is unlikely the IRS would assess any penalties for late return of a check that you have no idea what it is for. You should be receiving a letter explaining the reason for the check. Since the IRS is way behind in processing paperwork, it may take a while.
The only phone number is 800 829-1040. The IRS has started taking phone calls again but it will be hard to get through since they did not take calls for serveral weeks.
If you do need to eventually return the check, here is the procedure: https://www.irs.gov/taxtopics/tc161
