battery drain when car is sitting off

I just replaced the serpentine belt, alternator, and battery. replaced everything on a Tuesday, ran fine on Wednesday all day, Thursday ran fine in morning then after work in evening the battery was completely dead. got a jump start and it ran fine to get me home, battery light came on briefly along with airbag light but went off shortly after. have checked battery with meter and it reads at 14 when its running, alternator is fine and belt is not too loose or too tight, fuses test as good, connectors are good as well. I turn everything off including the radio and the heater, unplug the phone chargers and all that when i turn the car off. And when I turn it back on after its been off for hours the battery is completely dead. The old battery charged and tested fine when out of the car so we switched the batteries back out. but the battery light and airbag light are still coming on. What else should we be looking for? Going back out in a few hours to see if it turns on again.