Do you need to locate your septic tank? You can do that yourself, quite likely, without calling the plumbers. There may be documentation, in your “papers related to the house purchase” drawer, and you can spend some time searching there. Or maybe the previous owners can still be contacted. But if you can’t locate the paperwork, you could take some simple tools and venture out into the yard. An example of a simple tool would be a steel rod, perhaps five or six feet long. You will need to probe down several feet. Or you could try a metal locator. Start at the point where the sewage pipe leaves the house. Probe gently. You want want to poke a hole in a pipe and complicate the issue.
Your probing may find something unexpected, buried in your yard. Or perhaps you’ll find nothing. If you do find something you don’t understand, or you fail to find anything at all, consider calling in the professionals. Your local plumbing company will be able to tell you what you have and what you need as well.