It is kind of you to report it here. You could look up the Unit ID in Settings » About, and report it to Garmin's Customer Support - they should be able to find the owner in their Garmin Connect database. With a bit of luck, it can be possible to find the owner even from our (user's) side, so you can post the Unit ID here too, and I can try it.
Garmin respects and protects the privacy and safety of all our customers. We are unable to share registration information with anyone besides the registered owner.
If you have found a device, we suggest that you turn it in to local law enforcement or the front desk of the establishment where it was found.
So sounds like Garmin, if willing, has to deal with the owner directly without exposing personal information between you and the owner.
