I always hear/read about the speed trap warning. In motorcycle circles, IIRC tapping the top of your helmet is that same warning. I haven't ever actually seen either of those in practice, though.

Motorcyclists do usually wave at each other, or sometimes point at the road with a foot or two fingers, just as a greeting.

The one I do see (in the USA anyway) is flashing your high beams to let a semi truck know they have space to merge in front of you (eg after passing). They usually tap their brake lights a couple times to signal thanks after merging.