There are lots of ways to communicate, especially if you know morse code or have pre-arranged signals in advance. The problem is, not many people know morse these days (I’m very rusty on it myself).
The latter method was used in a lot of magician-assistant communication in magic tricks. (Example: assistant covers a coin with a coffee cup or mug while the magician is out. When he comes back in, he can tell what coin is underneath. Secret: the value of the coin is signaled by the location of the cup handle.)
One magic trick I recall had the magician and assistant communicating by one simply holding the other from behind by the temples. It looks for all the world as if you’re using telepathy. What you’re really doing is clenching your jaw muscles – something that can be felt at the temples. You can send more code this way, and no one is the wiser.
Spycraft depended on things like this, too. One way was supposed to be to use specific ways of lacing up your shoes (don’t just alternate holes as you go up). You can communicate meanings simply by appearing in the same place together. You don’t have to talk, or to even get all that close. Of course, you have to have pre-arranged what the different lacings mean.
I never really bought the “sending messages with a deck of cards” as used in Heinlein’s “Gulf”. It’s surprising that , without pre-arrangement, the other person can dope out what you’re doing. It’s not trivial to interpret, even if you do know, and setting up the message would take, I suspect, a suspiciously long time.