1 It's a singular piece of luck, not a general state. By empasing the singularity, you imply that it may well not happen next time.
2 I'd say 'before I left the bus I went back to get my sandwich, which I'd left on the seat, but I couldn't find it.
3 'What's this mean' is a colloquial form used because it's easier to day. It doesn't make much sense grammatically, but it saves on tongue work. It does stand for 'is', but don't try to analyse it grammatically.