When I left with two police officers.
So it looks like leaving babies alone isn't illegal. I told them there was a sleeping baby in the house, to explain why I was worried about keys.
(DS2 is 12 months. I was going to let the police into a house a few doors down, as the alarm was going off. I left the door unlocked, and the police knew there was a baby asleep in the house. No candles going, nothing on the stove, etc etc. I was gone for less than five minutes.)