Well I've heard of particles floating in the brake fluid, however, I have never heard of the notorious "sticker floating in brake-fluid resevoir" problem
. LoL that baffles me. It's beyond me how a sticker would've gotten in there - what was the sticker? It probably came off of something on the cap of the resevoir or from the can of brake fluid they were using to fill it up. As far as removing the particles, my guess would be that it's just old brake fluid, and that a brake bleed / fluid replacement should solve it.