Sorry to hear about the theft OP. I was wondering how they have gained access to get to the bike. Where I used to live, the entrance was on a timer so the postie could get in between 8 and 9, for example, otherwise they’d have to buzz each flat until someone let them in.
If you haven’t been followed home, then I’d suggest it’s either mates of someone living upstairs, or a tradesman who’s been working there recently. It’s not a huge stretch to imagine your upstairs neighbour sends photo of bike to dodgy mate, what’s this worth? Then lets them in to nick it when they know you’re not home.
Hope it gets back to you anyway, good luck.