n1ck,
I don't have an answer for you, just concurrence...because it sounds like I've got a similar situation. When winter rolled around, I had to have a 75 watt red light turned on 24x7 in order to keep the substrate surface temp on the cool side at around 76 degrees on average. I also have an UTH for the warm side, which keeps that end around 85-degrees.
Around the same time that I began using the red light, i started seeing a lot less of my snake. She used to come out daily and lurk around her viv at dusk (hunting time) for an hour or two. Now, I never see her unless I go in after her. The packaging on the Repta*Sun Red Light claims that it's barely visible to reptiles and recommended for evening use, but I'm not so sure based upon my limited experience. On the other hand, perhaps it's a seasonal thing, maybe they can feel the winter, regardless of the articial heat source and just prefer to hide out.
My snake does get natural sunlight during the day, so I don't think that her day/evening cycle is out of whack, but something is definitely different.
On a related note, to make matters worse (from a viewing perspective), I recently switched from a paper towel substrate to aspen chips, and now she can really hide 24x7 - I have not seen her crawl around on the surface since...she remains completely buried.