Re: Key? No thank you.
I did something similar once, by accident. I lost the key to my flat, and since the company I was renting from charged a fortune for replacement keys, I told the custodian i'd locked myself out and borrowed the spare key to let myself in.
A quick trace of the key was followed by some work on a blank with a triangular file, and I had myself a replacement key for a few bucks. I only discovered that I'd copied the master key for the entire tower block after coming home drunk one night and sleeping on someone else's sofa.
I woke up before that person and quietly let myself out. I dread to think what would have happened if I'd staggered as far as the bedroom.