I'll pass on what I still remember Dick Sanford saying on the R22 Safety Course.
In the event of engine failure, you will get a violent yaw to the left. That is apparently the most obvious sign, so if you get that AND the horn, then enter auto-rotation immediately. But what Dick said was that there are other reasons for low rotor RPM. The most common one is over-riding the governor. He gave a likely scenario...you're on a cross country, the weather starts closing in, you're not sure where you are and perhaps a bit worried about your endurance. You start to worry and get tense, which means you grip the collective too hard and inadvertantly over-ride the governor. The next thing you know is the horn goes off. Already stressed, you enter auto-rotation...
He said there had been accidents from unsuccessful attempted forced landings in this sort of situation. So what he told us was: if the horn goes off, with no other indications, roll on throttle and flare. The flare will raise the RPM anyway, thereby giving you slightly longer to see if it works, or if you really do need to enter auto-rotation.
The only time I've experienced the horn and going off in flight is during governor-off training. You might say that doesn't count. However, it is also a good thing to practise (with an instructor, not allowed otherwise), just so that you don't immediately panic as soon as the horn goes off. Governor failure is of course another possible cause of low RRPM, and I gather is not unknown...if something can fail, then one day it will.
I hope that helps. And if you haven't been on a Robinson Safety Course, I'd highly recommend it; one of the best things I've ever done.