I have a 2003 Civic EX.  I know it has an immobilizing anti-theft device,
but  thought it was quiet.  Yesterday, I had parked, and while I was
removing the key and getting out, the horn began sounding, in a pulsating
manner and the lights began flashing on and off.

I put the key back in and the alarms stopped.  The car acted completely
normally.

Has this happened to anyone?
                                            Bob