Mmm difficult one this.  Firstly because you didn't provide all the info (in your initial posting) and secondly because you had already committed your boss to returning calls when you hadn't actually spoken to him.

On this occasion I would have pointed out that he was due to leave for a 3 week holiday and that he may not be returning to the office that afternoon.  By doing so you wouldn't haven't committed yourself or your boss to returning calls or actioning requests.

There is always a chance individuals get delayed and do not manage to return to the office even though they have said they will.  However I reckon your boss was going to contact you but guessed you had already left for the day.  If you had left a voice message perhaps he would have picked up your requests rather than leaving a note on his desk.  But as I said, the last thing on his mind was actioning any requests when he was about to go on leave for 3 weeks.