Hi mate,
I have also had this problem a number of times, and it can be very frustrating. For me there is always this place where I used to park my last car, on the road next to mine, but since i got my IS200 sport I can't park on that particular spot anymore as it wont lock. However, as soon as I take the car further down the road it locks perfectly first time round, but then i have tried taking it back to that same spot on the next road straight away and it doesnt lock again! I have tried all the spare keys and gone through the usual routine, but its always the same. This has also happened when I go out in the evenings, and I have to move my car to a spot where the remote works!
I spoke to lexus engineers about this, and they say that all their lexus models have experienced similar problams, and there really isn't anything they can do about it. They said it happens because of interference with the remote/alarm system (which always worries me a little). I have heard of remote jammers and stuff, which block out ur remote from working totally, so I am assuming this is a similar problem.
I don't know much about the lexus security system, but I reckon if you live in an area where there is particular radio/electromagnetic radiation, it may be contributing to it. I know for a fact that I can't park on certain spots or anywhere in sight of those spots, as my car just won't lock! I have tried parking my mercedes there and it never has a problem!
If anyone has any suggestions, feel free to shout!
Rav