Ask HN: New to a system with maintenance in mind and no sparse docs

I have been hired a year ago to work for a company to maintain and update system that we do not have very good documentation for nor do I have insightful knowledge of the tools that are used to maintain and so on.

Wonder if there is a good rule of thumb to keep working on the system if there is a lot of time between support requests. Like writing additional tests, interference testing to find patterns of failure and so on to move to deeper understand it.