You might check this website dedicated to reporting stolen firearms. You might make sure it is reported there as well, or at least make sure the owner knows about them.
HotGunz Stolen Gun Search
https://www.atf.gov/content/firearms/firearms-industry/firearms-how-report-theft-loss
Thoughts?
What harm could it do? Take advantage of all the resources available to you that may assist in recovering what is rightfully yours.