The point is that expertise in plumbing isn't essential to the plot. Sometimes it might be, but sometimes it is just a background thing that a plumber would catch but most people don't. If the scene is in the basement a plumber might notice that the drain pipes are all flowing uphill, but the average person just sees a bunch of pipes that to think look like the pipes in any other basement.

Of course in a lot of cases expertise is needed. However that isn't important. The important part is you have more knowledge about something than the average person.