it's an awkward situation for the dealer - you describe the problem, which the car is no longer doing, and they try to replicate it and can't - so based on all of their diagnostic testing, everything is a-ok, so what are they supposed to do?
Electronic devices glitch all the time - how often have you had software crash on a computer, only to work just fine once the system is rebooted? Intermittent failures/issues are really hard to deal with - if you can't reproduce it, what do you fix?
The idea of documenting it as completely as you can next time it does it is a good one - that way they can at least see the issue in action, so to speak.
One thing I noticed in the manual which no one has mentioned here is the relearning procedure (or whatever it does) described in section 9 of the owners manual (page 9-20 and 9-21) - it says that's suppposed to help even if there are lots of conflicting devices/noise in the area (which I am guessing is the problem when there are two key fobs for different vehicles present)