This has more of an Urban Exploration feel and probably fits better somewhere else, but I'll give it a go here.
Serendipitously got invited into a closed and empty Grainger Market in Newcastle upon Tyne, UK, after the supervisor saw me in a doorway changing lenses and sheltering from the sun for a minute. Dark is my métier and I didn't see any reason to change that here. All with an Olympus P5 and mostly the 14mm f/2.5, maybe one or two with the 25mm f/1.7
The guy said I could have as long as I liked and in retrospect, I wish I'd stayed and tried to get that Grand Central Station shot with the sunlight streaming through the windows; felt I was imposing, though, so I left him to his market. Tremendous experience, though, and I was lucky to run into the guy (have since emailed to say thanks, I hope he got it).