Is there a scientific basis for why unit behind a glass should not be detectable on thermal?

There are three types of IR radiation: near IR, medium IR and far IR. At .74 µm - 1.4 µm, near IR is still close to the visible spectrum and usually travels through glass, especially window glass. Remote controls use this also - you can switch your TV on/off through a window.

Medium and far IR (1.4 µm to 1 mm) are often made visible with IR cameras, including FLIR. They do not travel through normal glass made of quartz. It requires special types of glasses to be transmissive to these types of IR. Even with these special IR optics, transmission values don't go beyond 70 %, so 30 % are absorbed.

I don't believe that for example MRAPs have their protective windows made from anything else but some sort of laminated quartz glass. Very thick and pretty much absorbing or reflecting 100 % of the medium and far IR. Only speed cameras make use of the near IR when they catch you speeding, but their range is very limited in comparison to high tech FLIR.

I have worked with a FLIR operator and it is true: You can't see through windows and you can't see, what is below the water surface.

It is pure hollywood, that you can see through objects (f.e. inside buildings through walls) with infrared sights.

You only can see heat emission with FLIR. If you find cover under or behind an object, that is warmer or colder than your body, than you are invisible. If you touch a window long enough, you will only see a handmark from the other side but not the whole body.

/upvoted

I'm afraid this is true, Flir cannot see throuh glass :
https://www.youtube.com/watch?v=9PLo5iCEcX4