Ok, so here's my take. I tried to make it sound as simple as I could. Please forgive the length of my explination. I studied thermo physics my first year as an engineering student and we took a look at how insulators affect heat signatures. We were studying whether or not the movies were accurate with those infrared goggles that can magically see through walls. This is possible as long as the structure's walls are not effectively insulated. If you are asking whether or not snakes see our heat through the glass, that would depend on just how well the glass or plastic is insulating against heat transfer. That being said, if they can see the slight temp difference between your chair and you through the glass, then they should also be able to see that same temp difference through wood or thick plastic which both have about the same insulating value as glass. In the case of a ball python they base their "picture" of the outside world on many senses. So for them to get an accurate picture of our heat signature the glass would have to have nearly no insulating value whatsoever. This however is not true, so what a snake senses in the cage is a uniform temperature across the surface of the glass. This of course is not exactly uniform as there is a temperature gradient within the cage from right to left as well as top to bottom. When your snake follows you around the room with his head it may be based a little bit on your temp, but probably it's mostly based on movement. Even vibrations in the air will be a bit muddled as the glass will vibrate uniformly across its surface and the snake will not be able to pinpoint the origin of the sound.
Sorry if this sounds a bit like a university class, but its how I think things through.