CDs have been proven to not work nearly as well as a real mirror. Better than nothing but not what I'd pick if had the choice.
Want to see a cool thing with a CD? Put it in the microwave and set it for 15-seconds and look through the window.
You will see a dancing lightning storm right above the CD, it looks cool.
PS don’t use a CD you value as it will be junk after this.
As far as using a CD to signal, I’m sure they would work. But they are somewhat breakable to pack around.