Do they suck up brain waves and hang around the Circle Line dock waiting for victims?

No, in the case of Freds, they shuffle through subway stations rattling cups to collect change, or lounge outside buildings with odd objects scattered around them. It's guaranteed to have them formally NOT noticed by almost everyone.

The Ralphs are more apparent. You find them in the form of middle-aged Chinese women who stand and say insulting things to your face in Chinese when you didn't notice a part of the sign for the price of individual lichees (don't want to buy a pound, just a couple? Offer a coin and have her point at a smudge on a taped cardboard sign and begin the muttering). Har! sounds different in Chinese, but I couldn't begin to type it for you here.

Off to the waterfront this morning.