We have done these types of surveys in the past and typically look for (as you mentioned) the location of the DWV pipe and the accuracy of the current as-built drawings. One of the projects we recently did has come back to bite us in that the plumbing contractor is having to spend an incredible amount of labor correcting slope problems in the building. Turns out the existing plumbing in the whole building is 100% sloped the wrong way. So, now instead of just replacing the pipe, he (the contractor) is haveing to re-route a considerable amount in order to get the correct slope on the pipe.
Given the conditions above the ceiling (with duct obstructions, conduit, etc), should the slope problems have been picked up on the original engineering survey? Keep in mind the original scope was just to replace the failing pipe.