Some time ago I bought a membrane keyboard protector for my home office computer, to keep dust and cookie crumbs out of the keyboard. It worked for that purpose, but for a while I have noticed that the membrane seemed to be sticking to the key caps so today I pulled it off to see what's going on.
UGLY!
There was a sticky, gooey slime on the tops of all they keys, and migrating down the sides of many of them (this keyboard has real keys, it's not an "island" keyboard). I've just spent two hours with rubbing alcohol, toothpicks, and Q-Tips, cleaning up the key caps. I haven't decided whether to try washing the membrane and putting it aside, or if I should just toss it.
The only thing I can think of is that the membrane has been off-gassing plasticizer, and combined with finger oils or whatever might have been on the keys to create a mega-mess.