Steve,
I have exactly the same problem with my Mk1 RX300. There seams to be no logic or reason why it happens. Is there a sensitivity adjustment i wonder because i have to say the alarm is sensite, for example try to open the tail gate without switching the alrm off firts and off it goes.
If you find anything out i would be greatful if you could let me know.
Regards,
Darron :tsktsk: