This is the most comprehensive debunking I’ve seen of the “stealth in space” concept: http://www.projectrho.com/public_html/rocket/spacewardetect....

This is one of the classic online nerd arguments, much of which is documented there. I’m a little terrified of reigniting the whole thing here and now to be honest, but if people want to get into it, have fun.

SBIRS is intended to use infrared to track ballistic missiles through all stages of flight, including decoys and debris released during the mid-course phase in space (so we know it's meant to be extremely sensitive, not limited to tracking hot rocket engines during the boost phase) Seems like a good bet that orbital systems with such sensors could be used to track LEO satellites too.