That is good you found the key!
Lately I have been using the round circle that you put your keys on ring thing
? (I'm sure it has a name but I don't know what it is)! to keep the zippers closed, it is secure and doesn't require a key! 
hehehe.... that round circle that you put our keys on a ring..... heheheehe a key ring?

heehehehe
I use S-Biners to keep my zips closed.

I know it's not theft-proof, but it is a slight deterrent.

I always had them on my pack (proper backpacking backpack and day pack) and a few times I'd find the zips in a different place then I left them or I'd feel someone trying to take them off....

Well, they can try, but it does take a second to take them off.... and that's if you know what it is!

I have black ones and they blend in with the black zippers, so it's not obvious there's anything there without looking closely.

I backpacked 2-3 years in Europe and never had any problems.
http://www.rei.com/product/811687/nite-ize-stainless-steel-s-biner-size-1-package-of-2(I might have a different size than this.... I can't remember.... but it's the same thing)