As already noted, lots of sound guys record everything they mix, even if they don't ask permission. Sometimes that stuff gets out. There are ways to surreptitiously stick a cable into something, but for all the reasons noted I wouldn't do it. With a digital board, you will get no signal from an out unless the engineer sends it, so that eliminates most house boards. For analog boards, you can certainly try and sneak up with a cable, but that is an extremely uncool move and will likely result in a crappy recording anyway.