I've read some people to say that they don't even know where their horn button is or if it works - they never use it.
I have a different point of view. I am quite willing to sound my horn when approaching a situation where I am not sure the other road user is aware of my presence. If they get offended, that's ok. They would probably have found something to be offended about in any case. If you scared them, then they probably didn't even know you were there. If you stopped them from doing something dangerous even one time in twenty - it was more than worth it. I am not saying ride around with the button depressed - but for me in a 20 minute ride to work, I end up using the horn probably about half the time at least once.
This morning's theme was the non-signalling but starting-to-move parked vehicle. About 15 years ago this situation occurred where a slightly-moving but essentially parked vehicle performed a u-turn with no shoulder-check or signal in front of my moving car and ended up with both vehicles seriously damaged and the driver of the truck pushed into the passenger side...not wearing a seatbelt either. As a result, I am always wary of parked vehicles that could pull out.
As I approached the car, the lights came on and it began moving forward slowly - no signal and no head movement from the driver. I honked twice (loudly - aftermarket horn) as I approached slowing down. The car stopped dead. As soon as I passed he immediately executed a no-signal u-turn from the parked position.
I can't be sure he would have done that in front of me but it would have seriously injured me or worse if he had. Don't be afraid to use the horn if you're not sure they see you....and if it's a wimpy little hamster horn, invest in something more attention-getting.