Strange things with my LS.. Parked car in evening, locked as usual, went home. Then after couple hours saw that headlights is on (was off before). Went to car, it was locked, no keys inside and ignition was on (headlight swich left on auto, that's why lights was on). Tried to unlock the car but car didn't reacted on key buttons. After couple minutes alarm started beeping, then unlocked car. Engine started easy, turned it off, removed the key, ignition still left on. Disconnecting and connecting back battery also didn't help. I think it's not first time because couple times I've found dead battery after night.
What do you think?

 

I think the key ignition switch is defective and still thinks the key is in the on position.

also, does the car have an aftermarket alarm system?

 

No alarm.
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And what year is it?

 

It's 99. Maybe is possible to check via obd diagnostic?