What causes a tube to become deflated on its own? I recently went to get my bike ready for a ride, and I noticed right away that my rear tire had completely deflated. I took off the wheel, and took out the tube and checked it for puncture but didn't find anything. So, I put the tube back in and inflated the tire. As usual I had the tube inflated to 120psi, the max for the tire. 

I went on a ride, it did not go flat on me. I put my bike away, and lo and behold the next time I looked it was completely deflated again. 

What causes an inner tube to deflate on its own? 

It also gets very hot in my garage. Does heat play a part?

I assume a new tube will do the trick. Is there a reason to worry, or can I safely ignore the problem and just inflate the tube every time?
Sometimes the puncture in an inner tube is very small and the air leaks out very slowly.  This is called a slow puncture.  The classic technique for finding a slow puncture is to immerse the partially inflated inner tube in water and watch for the appearance of any bubbles. 

In my experience even the slowest punctures usually get bigger somehow - I don't know how but I prefer not to wait until this happens so I'll usually fix the tube or replace it.