What could drain my battery with the car being turned off?

Since the engine does start then the problem is NOT the battery! .. Try wiggling the key... It may not be returning to the proper 'run' or on position after starting engine... Also try turning key slowly to the ON position but do not start the engine ... Dash components work then?

Sounds like the ignition switch to me also. Had a friend chase a problem like this for months and spent hundreds. Start small and with items known to fail like ignition switch. If the switch doesnt do it i would lean towards the ecm. They have issues that can cause them to still put power to things like the coil packs which will drain your battery and cause a no start after sitting over night.

Found out the hard way that leaving anything plugged into the outlet such as a car charger for your phone, etc does not shut off the power to said item when you turn off the key. So it’ll put a drain on your battery as long as it’s plugged in. I always unplug everything from mine now and have had no issues since.