They are called 'Flumbs' -
http://theframersforum.com/pmwiki/pmwiki.php?n=Main.F
And they are put there by the flumb pervert - a guy from another dimension that moves at amazing speed and with the ability to 'cloak'.
He carries bags of assorted flumbs to tackle all situations - white flumbs for dark mounts and vice versa - plus for flecked mounts he has shiny flumbs and for those with vucuum cleaners and compressed air he has self-adhesive flumbs. For those that thwart him too often he also has living flumbs - they look like a normal flumb but are actually tiny insects with indelible ink for blood.
He also carries all the tools and tapes required to undo the backs of frames, throw one or two flumbs in and re-seal it all in approximately .65 of a second.
You can't beat the flumb pervert; your only chance is to join him - by deliberately leaving at least two extremely skillfully placed flumbs inside each frame - there's even more chance of him leaving you alone if you advertise this fact and offer money or a discount for anyone that can find them within one hour - a bit like 'Where's Wally'
Walking backwards around your workshop three times, naked; chanting "Ég elska flumb öfuguggi" ("I Love the flumb pervert" in Icelandic) whilst throwing glitter about appeases him for about 7 hours. A ritual sacrifice of a Dyson appeases him for a whole month - but the daily walking backwards thing is much cheaper and no real bother.