Toad

That gap by the toad's head between the wood and the stone (?) is plenty big enough for it to get through. It's probably sleeping in the outbuilding during the day and going out in the nighttime cool to eat your slugs. I should leave it be. I had one for several years used to spend the day under a broken flowerpot, and hibernate over winter buried in the soil under a large planter. I only found that out when I nearly skewered it forking over the ground in early spring!