check the valve stem. The rubber one deteriorate over time and can leak, crud can get stuck in the valve and cause a leak, or the core can loosen and cause a leak as well.
I was thinking this as well. It could very well be the valve stem and it is rubber.
Also I didn't consider the mess that I would encounter when I change the tire if I use Fix-a-flat. lol
See that's why I come here. Thanks.