Is there a form we can file to avoid a penalty when our tax payment check was stolen from the USPS dropbox? We were contacted by a local detective who found the check.

There is not a particular form that I'm aware of, but you could attach a letter with your payment explaining the situation.  If you can also obtain a copy of the police report for the case number to include, that may be useful.  All you can do is ask for abatement of any late payment penalty.  

If you later get a letter from the IRS assessing a late payment penalty, send the same documentation as explanation and request that the penalty be waived.