Very glad to see that the OP's cat has been freed.
I expect the builder will have given the owners a heads-up already. If I were the OP I would drop in a note, and also keep an eye out for when they arrive tomorrow and go round there. Depending on what the wood floor has been treated with, it might not be too bad, and as the PP says, the rug is unlikely to be new if it's on a building site.
This thread reminds me of when I was staying with my mum once and woke to find one of next door's cats in bed with me, which gave me the fright of my life! We worked out he must have been under the bed all night. Luckily he didn't do any damage and we all laughed about it. :)