I always check my the air pressure in my tires before every ride. Today before I take off, I check the air pressure and sure enough the pressure is way down. I topped off the tire since I have slime tubes was getting ready to go (slime is supposed to seal leaks). I decided against taking off and replaced the inner tube with a spare I had in garage and went on my ride. When I got home I checked the inner tube and sure enough there was hole in the tube that was slowly leaking air (I guess slime tubes will get further down the road but won't completely seal a leak).

It would have been a real pain in the butt to out 10 miles and get a flat, even though I carry a spare tube and CO2.

FYI - This is first time I have changed tubes on a bike with thinner tires and I quickly found out I needed to partially air up the tube to keep it in the tire. When I got home I patched the tire and ordered a Topeak frame pump to use as a backup and to get me started when mounting the tire/tube on the wheel.

What tire pressure? Size tires?

I have found slime only holds (if it holds at all) to about 80 psi.

If I read your post correctly, you did not find the cause of the leak and remove it - ergo another flat (unless I misinterpreted your writing).

I stopped using slime many years ago. Also, it screws up your valves, at least for me.

Tire Pressure 80 psi
Size 700 x 32

I checked the outside and inside of the tire and did not find any objects (I did find a couple of small goatheads on the outside of the tire). I assume that I ran over a goathead and it punctured the tube.

OK - those goatheads can hide inside the rubber. I turn the tire inside out, applying pressure on the tread to let the rubber "open up" a bit, to possibly find the hidden goathead remnant. Same for radial tire fine slivers of steel wire. Some folks use a piece of cotton or tissue and rub it around the inside of the tire.