Hmm, as Saraj said, that sounds like it's not a master key
With a non-master, the security light comes on briefly and then goes out. With a master key, when you insert it into the ignition, the security light immediately goes out. At least, that's what it does on my IS200 and so far I've not had anyone contradict me on that.
You might want to see if you can get a Lexus dealer to confirm for you (hopefully for free since it'll only take them a few seconds) whether that's the case, since it's a rather expensive job (new ECU) if you want more keys and don't have a master.
Basically it looks like you're stuck with that key and won't be able to make duplicates