Keep in mind that a busy place will be filled with scores or hundreds of man-shaped thermal blobs. Not just the one your game shows you. The rest of the city did not suddenly drop to room temperature. So seeing a thermal blob in a busy place doesn't mean much at all unless that blob draws attention.
What about comparing what I see with thermos to what I see without thermos?
Well... I would first expect you to actually do this by looking at the place with thermos and without. Even then, in a busy room with some people coming, some going, some moving around and some hanging about, it will take time to tell if the man-shaped thermal blobs you see are visible to you without thermals or not. It will take time to link the man-shaped thermal blobs with the people you see without thermals. It's like those puzzles you do as kids where you take two very similar and busy pictures and try and figure out what's different between them. This will not be instant assuming it's possible to do it at all in a reasonable time frame.
What if I see someone hiding without thermals and a man-shaped thermal blob with thermals?
Good chance you can link these two as someone trying not to be seen draws attention and you can quickly check the thermal side and see they are the same. Or if the the man-shaped thermal blobs and/or normally viewed character are doing anything interesting or have something distinct about them. This all comes down to the fact that you have one more commonality you can use to quickly filter on: location.
In all, I just want people to recognize and consider the limitations of Thermals, especially in busy places. Chances are you are being meta letting your character come to conclusions that make little sense ICly. Thermals do not remove ambient population from consideration. If anything, it makes it MORE of a consideration as you see so much less detail when looking at someone through thermals.