Sssssssssssslow Leak?

This morning as I loaded the truck for the drive to work I noticed that the left rear tire was low, very low. Maybe single digit low. I plugged in my cheapo air compressor and topped it off. 29 PSI. I felt all around the tire, but did not feel anything. Today at lunch it measured 28 PSI. Did it lose a pound or was it just because it was colder outside on the parking lot that it was in my garage? I’ll check it tomorrow and if it is down again, I may have to actually jack up the car and take a real look for something.