Could the OP give a few more details? Were these no-shows people you already knew fairly well in person, or acquaintances acquired over the internet? While it is rude not to show up in either case, I can almost understand getting cold feet at the last minute about meeting up with someone you've only met online.
With the latest event event I had to figure it out myself when I showed up at the hotel and there was no room booked (and none then available, either).
With so much money invested in traveling, shouldn't you have asked the person to send you a copy of the hotel confirmation before you left? Assuming he/she was the one making the arrangements, of course. If you don't get a copy, that should probably be a major tipoff right there that all is not well.