He may not have turned you in, by name or by face, and he may not have turned you in specifically to your landlord, but he could have. And the coincidence suggests that the word went out and that your landlord took action in response.

As for the "master" key thing: Master keys can often be made by filing off a tiny bit of a non-master key. That's how they work. So, when you requested a copy of your key, you may really have raised the suspicion that you were about to go home, take out a file, make a master key, and go on a fun round of housebreaking.

(Obviously, you would not turn your original key into a master, because then when your landlord or the cops asked you to produce it, it would be tampered with.)

Incidentally: Because master keys are variations on the shape of non-master keys, your key doesn't need a "serial number" or embedded chips to match it up with other keys from your building.