Refleks said:

Either way the thermal signature of the person holding the rifle is bound to be way larger than the reflective signature of the optic, and thermals are passive.

Unfortunately, thermal can now be fooled using not very expensive materials.

gatnerd said:

Would a single lens Red Dot Sight (like the RMR) not be effected then?

It's a possibility that non-magnified sights may stay undetected, which is not a real problem because they are non-magnified (and so not used for observation and long-range shooting).

All systems i know of, look for multi lens optics, Its not meant to discount holographic signts ,its just a way to filter out all the unimportant glass and lights in the enviroment.

By the way  similar systems used in chasing ''pirates'' recording a videos in the theatres.