A week ago the tire went completely flat. I inflated it and took it to the tire shop. They found nothing and after checking twice, suggested that most likely some ne’er-do-well had let the air out of the tires. Things were perfect until the yesterday evening, when the tire was once again flat as a pancake. I inflated it (without moving the car) and set things up so that I could detect if it was a person who let the air out. This morning it was completely flat again, and this time I knew that nobody had set foot near the car. I inflated the tire again, listened closely, and heard a quiet hiss. I moved the car a foot or so, and no longer heard the hiss. I instructed my wife to check the position of the tire when she parked and make sure the valve stem was not at the seven o’clock position (where it was when we found the tire flat). Everything was fine all day, and when I checked the pressure, the tire had not lost air. Then I moved the car so that the tire was rotated back into the exact same position as it had been last night when we found it flat. I once again heard the hiss, and after waiting a few minutes and rechecking the pressure, I confirmed that the tire had once again lost pressure.
What could cause this to happen?