So I left my bike sit for about 2 weeks because I was out of town. I came back and was going to go on a ride. Turned the bike on and went to back it out. I couldn't move it, rear tire was flat. I used my inflater kit for my car and pumped up the tire. I inspected the tire and found no signs of puncturing and couldn't hear air leaking. I left it there for about another two weeks and the tire pressure is holding. What would have caused my tire to go that flat if it wasn't punctured?