I had my suspicions confirmed just now: my cat has the ability to put my MBA to sleep just by being in a certain location.
The pet door fitted to the outside laundry door has a magnetic catch activated by a small round magnet that dangles from the cat's collar.
If this magnet comes close to the enter/return key on the right hand side of the keyboard whist said beastie is being friendly, then the Mac enters sleep mode.
As this Mac laptop lacks an actual latch, I'm assuming there are corresponding magnets in the lid of the thing otherwise I can't imagine how closing it triggers sleep mode.