Q: What would cause the battery to drain. car does not start when it's warm.

Hello. There are many things that can cause a battery to drain even when the car is off. If any live wire on the vehicle, there are many, short to ground then they can complete a circuit that will place a parasitic drain on the battery. This will eventually result in a dead battery. I would carefully examine the vehicle’s wiring harness in the engine bay for any signs of damage that may be a potential cause of the issue. If everything in the engine bay appears to be fine then the issue may require a more in-depth diagnosis of the battery from a professional technician. In regards to your starting issue. It may be possible that the starter or starter solenoid are going bad. There are cases where the components will not function if they get too hot, but will work fine once they are cooled off. I would test the starter and solenoid to see if they are within spec and replace them if they are not.