At night, and particularly as weather cools, flies look for a place to roost. This applies from autumn to spring also. The crevices around openings, in between layers of material, etc. are favourite locations and they will crawl into the tiniest crevice. This keeps them safe from predators such as birds, lizards, etc. When the weather warms up (or dawn breaks) the flies warm up and go on the move again. It is common for them to move in the direction of warmth and light, if any, and they frequently go in a different direction from whence they came (a fly's memory is notoriously short). This regularly results in closed spaces filling with flies because they are not truly closed (lots of small crevices) and the flies can't find their way out. What you saw sounds like bluebottle flies and with the warmer weather I am not surprised they were there, a few (say 10-20) is not a high number, really, and when buzzing around in the heat of day they will seem very numerous. While it is of course possible they are feeding on something within the structure, I think that would have to be proved and my guess would be that they simply crawled in from outside.