ONLY 2 ways to answer your question. These are the only way excess water can get into the tank from your last post.
1. You have a sewer flush and as a BAD idea you have a Y water connection between the City water fill and the Black fill and the cut off valve to the Black is NOT shutting off 100 percent and is leaking water into the tank. I always advise customers that do this to install at least 2 cut off valves to make sure the one if it leaks does not fill the tank.
2. Your particular floor plan has a sink or shower that drains to the Black tank. There is NO code that stops plumbing a Gray fixture to the Black tank. Doug