Three years ago we had two nicked from our stair (this was before I took up cycling) - one was my neighbour's £1000 road bike which he'd left out for less than 24hrs. Now - we live at the top of 4 flights of stairs, and the bike would not have been visible from the bottom. It was either a very opportunist thief who chose that one day to get in and drag himself all that way up just in case, or - as we suspected - the postie may have been tipping people off... He'd have been the only one to come up to the top floor the day the bike was out and seen it.
A while after that, we had someone apparently come in and knock out one of the railings (plus a big slab of stone stair) to pinch a bike that - again - had been locked up there for less than 24hrs. Despite the fact that there were people in all afternoon in the flats beside where it happened, no-one heard a thing. And once again, another opportunist thief who struck lucky or someone tipped off ?
That's the reason I've always carried my bike up and down every day and keep it in the flat, despite the fact that it's knackering to do ! Never mind, I took possession of my new Giant SCR2 road bike today

- that's going to be like carrying a feather compared to the hernia-inducing MTB.