I have seen several similar threads on the warning buzzer, but my problem is different. The warning buzzer goes on even when the car's ignition is off. The battery went dead and I had to jump start the car. Once the cables were connected, the warning buzzer started. The car started and the buzzer kept going. No warning lights displayed and the car just idled for 15 minutes. No problems or other issues. I shut off the ignition and the warning buzzer kept going. I had to disconnect the battery to get the buzzer to stop. Any ideas?

 

Check coolant level at radiator, and power steering fluid. I believe the power steering computer has a buzzer too.
Also, does the door light come on when you crack the door? Might be a faulted door ajar switch.
Did you change headlights? Could be a headlight circuit error.
Possibly ignition cylinder error (thinks there's a key left in ignition when there isn't)
How quickly does it kill the battery? Makes me think there's a short circuit somewhere

 

Oh and also oil sensor (I believe the one on the side of the oil pan)

 

Coolant level is good. Engine oil level is good. Power steering fluid level is good. Door light comes on when door is opened. No recent changes to the headlights. The battery went dead after a month or so from none use of the car. The warning buzzer has not gone on anytime recently until just the other day. I am confused by the buzzer going on even though the ignition is off. Yesterday I tried connecting the ground cable to the battery and the buzzer immediately started. Car doors were shut and the key was not in the ignition. I had to disconnect the ground cable from the battery to get it to stop.

I just connected the ground cable to the battery again and buzzer did not go on. I left the cables connected to the battery and I am waiting to see what happens.

 

Is it possible you didn't push the key in to lock position before removing it? I know on my mine the cylinder is worn and it will let me pull out key before lock position, and it's actually kinda difficult to get to lock position.
I've noticed that sometimes (not always) the buzzer will go off, but I can't rely on it so I have to always visually confirm I've gotten to lock position.