Bicycle detail

I keep my bike locked to a tree in the backyard of my apartment. When I got home from work last night, I improperly locked it around the handlebars where it was prone to being slipped through and taken. Last night, I heard two loud taps as if I heard someone knock on either my back or front door. I checked through the windows first but didn’t see anyone at either door. I went outside this morning and see my bike was taken. The lock is still around the tree, and there was another bike left.