A very large, odd truss was observed in the attic of a very large old mansion, the function of which was unclear, as it was not supporting the roof to any significant degree (only attached to the roof at all by a small 1x6 board), and spanned from the front exterior wall perpendicular to the 2nd floor ceiling (attic floor) joists to more-or-less the center of the structure. A plumbing vent pipe passed through the center of the truss, but I did not observed any framing having been cut for the pipe below.
The attic was symmetric, and no similar truss was observed on the other side. There was a 3rd floor room in the center of the attic, with a small bathroom that was added on the side of the truss, the outer corner of which just overlapped the inside end of the truss.
The only purpose for this I could come up with was to stabilize the 2nd floor ceiling (attic floor) joists and/or walls below. Some sinking of the 2nd floor was observed on the rear of the structure, which the truss was obviously not intended to reinforce, but did not appear to be causing, either, no such sinking of the attic/3rd floor being evident.