Time flies...

... But I just got an mail from Marriott Business Services (Customer Service) which has investigated this. They say that the hotel claims that I indeed was there on that day and checked-in.

This leads me to this line in my OP: "But is this a glitch or is this scary? Could they have checked in another person on my canceled reservation and had this person charge my card?"

Seems that the hotel let somebody check-in on my canceled reservation that day, or? Later this afternoon I'm going to dispute that charge on my credit card. This is outrageous!!!