[68C15]

you had door open which kept the modules in car "awake". Anything not at rest on late model GM will keep the whole car "awake" and run the battery dead in a very short time. Leaving even a map light on will do this. Depending on year-make-model-options it can take anywhere from 5-45 minutes for the car to go "asleep" after all it's "wake-up" inputs shut off.

when you hook up a battery, you are sending the car a "wake-up" command.

what I do when working on a car and need the door open is latch the door using a screwdriver. Assuming the car doesn't have a door jamb switch this will let it go "asleep".

checking for parasitic drains can be tricky on modern cars. if you disconnect the battery and hook up your meter you have just woke up the car and will have to wait for it to go asleep. I install a jump battery to a constant power source (cigar lighter port) and then disconnect battery. You can also prevent further wake-up commands by not pulling fuses. Instead use a voltmeter and check fuses for any voltage (20 mV for example). To use this method you test from positive terminal to fuses just like doing a voltage drop test. You have all the doors open and latched before test, right?

if you ever do a parasitic test on a Ford product you will spend all day unless you make absolutely sure all the wake-ups are turned off (don't forget that each door has 2 switches)