The final step in any functional test should be the return of the system under test to a normal and safe operating state. It is important to allow some time in your schedule to remain on site and around the system you have been testing after the test is completed.
It can take some time, perhaps even an hour or more, for a system and its supporting utility systems to return to a stable operating state following test sequence, especially if the test sequence involved a lot of upsets and/or the systems and subsystems are large and have a lot of inertia. If something does go wrong immediately after the test or immediately after you have made an adjustment to a system, then your knowledge of the system, the test results and process, and the changes made will be invaluable aids in solving the problem or stabilizing the system.
The bottom line is to simply be courteous to the people who must operate and work in the building. Let them know when you arrive, what you are going to do, and when you finish. Hang around to make sure that the results of your effort are truly having the desired effect. If it turns out that there is a problem, you will have a client who appreciates your responsible attitude and courtesy.