I have the same exact thing. For a while, it was driving me crazy trying to figure out what it was, but I narrowed it down to the fact that I either cross my legs (somehow putting pressure on the above-knee area and leaving a temporary red spot), or if I was leaning over, my elbow would do it.

It's probably a similar thing happening to you, especially if it's not itchy or sore, or like any other skin condition. Good luck!