I must admit, I did not think about that it might still let IR light through. I got an old large tv and a fireplace whith see-through glass, I covered these with blankets and the bookcases and wardrobe with sheets, I thought that some of the movie containers might be reflective.
Is blankets not enough either?, because then my guess would be the tv but it is wall-mounted and very difficult to take down, so covering/blocking it would be prefered, if it is the tv. The windows are already covered by curtains, otherwise I removed everything I thought might be reflective, foto-frames, the lamp-shades made of some kind of metal and some decorative stuff and it was still the same.
So I guess I should try getting some curtains to screen off the tv and fireplace next?