Just the one battery.
Are you saying this red light for the OnStar stays on even with the key off and that might be the cause of the battery drain?
If so, remove that OnStar fuse and see if the drain goes away.
If you are sure nothing is being left on, then you need to test for a drain.
Disconnect the negative battery terminal and place an ammeter inline with the negative test lead on the negative battery post for correct polarity. Then turn everything off, remove the key, close the doors and lower the hood enough to put out the hood light if it has one or just remove the bulb.
Now set the ammeter an DC amps and it should not read over 50 mA, that's 0.050 of an amp. Up to 50mA is allowed for anything with memory such as computers and radio presets, etc.
If it reads over 50mA than it will eventually drain the battery, the more that it is over the faster the drain. To find out what is causing it, start removing the fuses one at a time and then look at the meter until it drops safely below 50mA. Leave the fuses out and push in on the door jamb switch when looking at the meter if you must keep the door open.
Then when you determine which fuse's circuit is the problem, hook the battery back up and try everything electrical on that circuit to find out what don't work and might be shorted, then use a voltmeter or 12 volt test light to trace the circuit from the fuse to the part that failed searching for where the voltage goes from 12 volts suddenly to zero or whatever and there is your short. Repair or replace the wire, part, etc.