I can't remember the context, but it's been pointed out that some low-TL locks are just too
big for normal high-TL lockpicking tools to work on them, though they're trivial to open once you've got the big tools. Worth considering in a cross-TL game.
I'm glad to say that master keys seem to be on the way out at long last. They've always got into the hands of criminals, and the recent
TSA lock incident has shown how easily they can get into the hands of everyone else too. (I won't link to it here, but there are tested 3d-printer-ready files for all those keys...)
Which leads to something worth considering in a TL8+ game of lockpicking: a casual photograph of a key is enough to duplicate it mechanically. If your covert observer has a decent camera, he can lie up, photograph the guy getting through the secured door, and have a key of his own within minutes. Countermeasures to this include magnets embedded in the key, or electronics.