Mirrors absorb more light than it reflects.
On a very small (think particle) level - mirrors do absorb light but it is misleading to say mirrors absorb light. I think this got started with the Cannabis bible or something and this is inaccurate.
The problem with mirrors is they are "too perfect" light reflectors. They suffer from something called specular reflectivity or "hot spots". Mirrors focus light and can burn holes in your plants (and anything else the light focuses on).
Bottom line - don't use mirrors (I guess you could glaze them with vaseline or something but i wouldn't risk it).
If you want to experiment for yourself regarding mirrors and lights you can try this - turn off all your lights at night and see if your mirrors seem to glow with ambient light. If mirrors absorbed light there wouldn't be any ambient glow but this simple test will show they do seem to glow.
anyway - sorry if this sounds nit-picky but i'd like to see the editors of the cannabis bible (or wherever this myth started) update their information.