one of your solution's "projects" could be a compiled .dll

Of ASP.NET: When opening a new solution and ramping up on its ways, check to see if some of the references seem to be custom in-house libraries. I've seen a few solutions now wherein something I would have kept as a project within the solution has in fact been tucked away into a referenced .dll. I'm not in love with this sort of thing personally as it just makes the whole of the "solution" harder to understand, search, and navigate, but it is something I've run into twice now.