Well, sort of undercover, as everyone in the apartment house knows who he is! This is probably the best episode so far of the first series of Rupert Davies as Maigret.
Shows him being observant, and getting inside the heads of the suspects. Eventually he works out why a few things don't look right, and sets a trap.
I saw these shows when they originally went out (early 60s) and they were very popular. I've now read many Maigret books, and can understand how hard it was to adapt the stories to fit into an hour show (later series are 90mins, or a whole 2 hours).
They were also made when TV was in its infancy, not nearly so much to watch, and for adults it was only on in the evenings! But this episode worked well, some very good acting (Stratford Johns in a small part, just before Z Cars!), and a plot where you could see the clues as the thing unfolded.
As an aside, I see that one of the minor players, a glamour girl in one of the rooms, was soon to be George Cole's wife. She did appear in a Minder, but I can't recall if that was actually as Mrs Arthur Daley (the famous "er indoors"), or something else not his missus?!