Hi , I am a new "old" Lexus owner and new to these forums , and I am sure I am late to this , but I wanted to share my experience. Last night I accidentally left my 2001 RX300 lights on (actually, I'm blaming my mechanic who left the lights in the 'ON' position and I didn't notice that, so I'm guessing that's what kept them on). So the battery was dead in the morning, and being a new Lexus owner , I wasn't familiar with how a Lexus behaves when the battery was dead. (different from other vehicles) . The security light was flashing fast without the key and slowly with the key. I thought mechanic had enabled something I needed to de-activate. It was wasn't at all obvious to me that my battery was dead. I thought my alarm just needed to be deactivated.
So I talked to someone who suggested my battery might be dead and that I try charging it, but as "Bullmtnbiker" mentions, the horn blasted as soon as I connected the negative terminal of the battery to charge. Finally, I just tried connecting it and running into the vehicle to start it, and that stopped the horn blasts immediately and it started and ran fine. I just hooked up to a trickle charge overnight to get the wounded battery back in shape.
So , if your Lexus RX300 battery ever dies, expect the horn to blast when connecting the cables before you can get into the vehicle to start it. Kind of an odd anti-theft setup in my opinion.