Whodunnit!!!

I have a mystery on my hands. I spotted what I thought was a dog’s ball in the front garden from a window this morning but when Martin went out to investigate it was a huge garden snail!!!
Now I’m not over fond of snails and I hate their sluggy friends but this one is quite cute. Except he’s not mine!
So if anyone can shed light on how he came to live in my garden I’d be grateful.