I've only occasionally been aware that someone at an airport/on a plane was deaf (or signing) - usually because they weren't travelliing alone.

I commonly come across people signing on the London Underground (well I used to when I could actually use the system - it's not exactly wheelchair friendly ), occasionally on the buses.

Every now and again I'll come across a group of deaf visitors doing the rounds. I'm quite shy, so I merely note their presence... not my mother. She tends to make a beeline for them (she'll do the same for other groups, it's not because they're deaf, she's just incredibly gregarious - recent article about her here - it's pretty typical). I've been with her on a couple of these occasions and I have to admit it's been lovely communicating with Italian and Spanish deaf tourists - speaking Spanish actually does help when communicating with deaf Italians just as it does when communicating with hearing Italians.

If you come to London - be warned!