Complete Mystery!

First post on the forum and I am asking for advice/help.

I found mice droppings in my tall kitchen cupboard so I purchased two plastic mouse traps the ones that are pre-baited. 

I had been told to use peanut butter to bait used traps because mice love it. Any I set both traps and put one on each shelf in the cupboard. The next day one dead mouse so set the traps again baited with peanut butter. The next day a trap had been triggreed but to my surprise it was a large brown slug!

i cleaned both traps and set them again with peanut butter and surprise surprise the next day both traps totally clean of peanut butter.

So I decided to bait the traps with peanut butter again but pour a thick line of salt around both traps. Next morning no salt disturbed but wait for it..... both traps completely empty of the bait/peanut butter!.

There are no new mouse droppings so what's eating the bait?

Please Help!.