This isn't normal

I just popped out to the shop. As I was leaving I noticed a set of keys on my outside table. They weren't there half an hour ago because I was sat out there and I would have noticed a large bunch of keys in front of me.

Who's been in my garden and why have they left a set of keys on the table next to my BBQ?

I'm confuzzled. The keys are now indoors and if someone wants them back they'll have to knock and explain themselves. That's a fair call isn't it?