I'm alone in the house- DDs are at school (and can't sneak home, it's over 2 miles and through a security checkpoint!), DH at work.
DD has a collection of ceramic animals that live on her window sill.. but I've just gone in to put her dressing gown away and found them in a perfect circle on her floor. I'm sure they weren't like that when I was upstairs an hour ago.
Logically, I just didn't notice them. But... I've seen Toy Story, who knows what they get up to when we aren't around...