Yes I am having the same problem with my MKII GS300SE. The alarm goes off, but no flashing indicators and no horn, just the siren. Tried switching of by pressing the unlock button on the key forb and it still goes off. Started the car and drove off, alarm still sounding. Finally pressed the lock and unlock, it stopped for a while and it then came back on. I have now disconnected the battery and will phone my lexus dealer on Monday.
If i find anything, i will post here!!!
Does locking the car using the key in the door diable the alarm????
SID!