I know lots about slugs Dick. Short answer - they came from underground and have been yomping away on some plant life or vegetation. Now the long blurb!

Nematodes are the answer. They kill the nasty beasts underground without harming the environment.
Whatever you decide, don't go out with the big guns just yet (i.e. blue slug pellets which are harmful to pets and birds). Look up a local or internet supplier for nematodes. We use a brand named Nemaslug in the UK. I'm from the rather wet (slugs love wet weather) Isle of Man in the middle of the Irish sea.

There are nematodes for most garden predators, but slugs can eat lots and lots of your lovely plants, particularly freshly sprouted veg - and practically any other plant with lush new green growth. Hostas are another favourite.

Actually, the really big fat brown slugs prefer rotting timber and leaf mold, so would not do as much damage as the black and lighter brown smaller ones.

Perhaps you have recently created an area they love (under stones, plantpots, practically anywhere dark and damp. Or maybe you have started growing some lucscious plant they are feeding on.

I use nematodes each year - clever little bacteria you just mix in your watering can and water with a large rose. I only use them on my veg patch, so a 40 sq metre pack which comes at six week intervals throughout the main growing season (end March, mid May, end June). They cost around £24 for all three packs.

Good luck, tho I imagine you won't have nearly the slug population with a much drier environment.