Strictly speaking, they always have to signal it. However, you might notice the coaches will grab one of the officials and let him/her know that the coach will want to call a timeout. This is especially true when the coach is deliberately letting the clock run down. The coach may even say that they're planning to call a timeout with X seconds left, but they still have to actually call it.
So to answer your question directly, the coaches always have to formally call a timeout and make the signal; they can't tee one up and rely on an official to call it for them.