Have you heard about Adam Savage (Mythbusters) accidentally getting through security then realizing he was still carrying a sawblade on him?
The TSA announced random security checks here. Everyone pointed out how ridiculous they were but they still went ahead with them every few months. I assume they are still going on as I have yet to encounter one.
They take place at a (supposedly) random station when done. You would have to be VERY unlucky to be caught.
The worst that could happen is they ban you from the station that day. You could just walk a couple blocks to a different station or even another entrance to the same station.
They randomly (supposedly) pull people out of the faregate line and ask you to go to a table. Of course you can refuse.
They do their oh-so-thorough check of your belongings. A smart criminal would just hide stuff. Or if they were a bomber and for some reason did not opt out set off the bomb at the crowded security table.
I forgot what were the other details but it was pure comedy that anyone thought it would do anything other than give a few people warm fuzzies while everyone else was delayed getting to their destination.