Are you across the room during the "period of non-use," or are you at the computer? Any chance you're between a light source and the MBP's ambient light sensor, thereby shading it, and that you unconsciously shift position, thereby unshading it?
My understanding is that the sensors were at some point (not specifically uncovered by me during modest research) moved from a location near a speaker to a location near the camera; the latter can sometimes allow low-trajectory light, which doesn't really illuminate the keyboard, to engage the sensor, thus dimming or shutting off the keyboard lighting.
