It is simply not possible to determine the cause of your flats from your riding habits, type of bike, tires, tube or what you did not find. The ONLY way to determine the cause of a flat is to find the puncture, look at where it is and what it looks like.

First remove the tube, but mark it so that you know the relative position of the tire, tube, and rim in relation to each other. For example, I always mount my tires so that the pressure info is next to the valve stem and on the right side of the bicycle. When I remove the tube I put an X on the same spot on the right side of the tube next to the valve with permanent magic marker. That way when you find a leak in the tube you can match it to a specific spot on the tire or rim.

Next find the leak - tub of water, soapy solution, feeling for it next to your cheek, whatever is necessary or works for you. Also check the valve by putting some soapy solution on it. Once you find the leak the type of puncture will tell you in many cases what caused it. I'll try to upload a drawing of the different types of punctures later, but the big thing is location. Match the location to the same spot on the tire (or rim if the puncture is away from the road/sidewall surface. Look for a cause....more later.