My wife and I gave our old car to her parents. In the subsequent years, its battery kept dying and we had to go over to their place and jump it. Never happened in the prior 7 years we'd owned the car. The mechanic said nothing wrong with the battery, but suggested there might be a parasitic drain somewhere.

A few days later, we were visiting them. I looked at the car and saw the door closed, but not properly latched and ever-so-slightly ajar. You know, that half-closed thing car doors do when you don't close them hard enough.

It turns out someone, possibly my son, had been not closing the door completely. This did not keep the lights on or cause any other visible sign, as it would in some cars.

After I told them to slam the door fully shut, never a problem again for 2 years and counting.