I left it in the hall, forgot to bring it in, and the next morning it was gone. I live on the fifth floor, and security generally isn't a big issue, but we've been having problems lately.

I have a 16" Brompton, so I'd like to go with something bigger. I don't think I can go big than 20", due to tiny apartment. Light is good only because I have to schlep the thing up the stairs.