I've not seen an adder, Britain's only poisonous snake, while paddling in Scotland. But this would suggest they're around. Lurking.
Or is it a subtle ploy to keep visitors away? Should we read the sign as saying "Get off my land"?
Perhaps I'm being paranoid. However, I once worked with a cameraman who put "Live Snakes" stickers on his equipment boxes to discourage thieves from breaking into his car.