One thing I noticed, the lock/unlocking of the car reacts differently depending on where the key fob is on my person. Allot of times I have fob and my cell phone in the same hand or pocket. That can definitely cause RF interference with the key fob. I have also found times when I go to enter the car and it does not unlock, but when I turn my body a bit, it than works. So must also have something to do with the antennas in both the car and the fob.
Don't know what frequency they are on, but the higher the frequency (like 2.4/5.8 GHz), the more interference it will experience from objects. Lower frequencies like 900 MHz which is used on RFID devices are less effected by that type of interference.