Magic Elves?

'Andy, there's a new bike outside, come and see!' Nathan and I tried to get Andrew's attention away from the paper. 'What?' He asked. 'There's a new bike, somebody replaced your bike with a new one.'
The family walked out to see the phenomenon. Andy's bike had been stolen a couple of days earlier. On the bike rack was a rusty, black Avanti. There was a new bike seat cover on the seat.
On the tyres was pieces of cut grass and dead leaves. We were standing there making comments when Andy said, 'Where's Lydia's bike?' I made a kind of stifled sound and looked at the new bike. When I had left my bike there after work I had left it in the same place that this bike now possessed. I looked round the drive way. Realization dawned. 'This is insane.' I declared. 'Why would anyone, if they wanted to steal a bike, replace it with this bike? This is two bikes gone now.'

Good question. 'The person who took it was probably drunk.' Andy said with conviction.
Nat voiced what I felt, ' A drunk person would more likely just steal the thing, not leave you with another bike.'
Were there aliens around? Or maybe some magic elves?
Walking is good for you I supose.