Since it's a Mavic Mini which doesn't require registration in U.S. as it weighs less than 250g, most probably it doesn't have a registration number because there was no need to register it.
I read in a post somewhere that there are pilots who save a text document in the drone's memory card containing their contact information in case it's lost and somebody finds it, will have a way to get back to the owner. Good to do this to the Minis, since they are not registered, it'll have the owner's contact data in case something happens.
Also, I think the Mini stores a big amount of technical data and large unknown files on the MicroSD card, that could be flight data which may contain somewhere information about the DJI user account, according to the pilot log-in into the DJI Fly app. So it may be possible that DJI or perhaps an IT expert (not to say a sort of "hacker"), has a way to find in those technical files who was the user of that drone, if it's the case that such information is saved deep inside hidden in those mysterious files. Who knows, that may help.