Perhaps you or someone else knows, but I frequent the https://tar1090.adsbexchange.com website often and I see unusual tail numbers listed and I can't figure out why they are listed as such. They may not even be military aircraft but just small aircraft.

The tail number in this example below is _AE0B60 and registration is n/a

Here's an example screenshot of what I mean https://imgur.com/a/A1p4N2c

Does anyone what this is, or why they don't have 'normal' registration info on adsb?

That's an ICAO/hex code. The transponder ping always includes that information, and then tar1090 tries to look it up in a database for other information, like the registration. If it can't find it in the database, then it just displays the code.

Codes that begin with AE are military aircraft, which don't appear in any official (e.g. FAA) database. Enthusiasts have built somewhat ad hoc databases that include many, but not all of the military ICAOs.

Those are hex codes - it seems to show that when it doesn't know the reg or an aircraft doesn't report one

http://www.milradiocomms.com/search_mil_hexcodes.php?type_of...