Many games are held online. In person, computers get smaller all the time (and there are bathroom breaks etc.). The history of trying to cheat at chess, casino games, and more with concealed computers is far too large a topic for one reply.

We had that story[0] a couple of years back, about the guy who supposedly cheated by using vibrating anal beads.

0. https://news.ycombinator.com/item?id=37657925

That didn't prevent Anna Rudolf being accused that she hid a supercomputer in her lip balm box made of tin sheet metal in 2007 and cheated with it in the bathroom.

Some parts of the chess community are very ridiculous.