Since it's only showing on the map, it's probably a VFR flight. Sometimes controllers will only put in the last 3 characters of an aircraft's tail number when flying VFR, so N[something]5MK and N[something][something]5MK will appear on your aircraft's map.
Shame on those controllers!
I'm wondering if FA has ever considered using some of those search routines that would match the last available characters with a database search of all aircraft tails ending with those same numbers, seeing whether any were last located in the appropriate state, and suggesting that the 'correct' tail number was 'such-and-such'. Presumably this would be done when a flight filed no flight plan and would be VFR with flight-following.