Yeah, I just don't even allow "taking a 20", no matter how much time or space or security the person trying has. I can tolerate taking 10's given good circumstances, but taking 20's is just pushing it too far in my book. But hey that's just IMO.
Really? I see taking 20 as a blindingly obvious rule, much more so than taking 10. Taking 20 just means that, in the situation where the player says "I'm going to try over and over and over until I roll a 20," you cut out the formalities. The player will eventually roll a 20, so it's pointless to make her physically do it.
The alternative to taking 20 is to say your first roll is what you got, you never do better. However, it seems weird that a quick once-over of a room is just as likely to succeed as a minute, painstaking examination taking twenty times as long.