Verifying integrity of septic system before selling home

A few years ago, when my husband and I sold our home, we were required to complete a home inspection.  It was necessary to perform certain tests to prove the integrity of the roof, electric, and plumbing system.  One of these tests was targeted at the septic system. A licensed plumber came to the house and introduced an orange dye into our drainage pipes.  If the orange dye appeared in the lawn behind the house, it would indicate a problem with the septic. Since we had never had any complaints with our water pipes, drains or septic, we were quite confident in the results.  However, when this process was performed, my husband and I were a little nervous. We really didn’t want to get into the work, expense, mess and aggravation of septic issues. Even a minor problem would require digging up the backyard.  I was extremely relieved when there was no sign of the dye in the grass. The sale on the home went through smoothly. When my husband and I were moving out of the house, I got talking to one of our neighbors. This neighbor is in his late seventies and has lived in the neighborhood his entire life.  He informed me that the septic system to my former home isn’t located in the backyard. He explained that the person who originally built the house had formed an agreement with several of the neighbors to share the expense of a single, large septic. He pointed to an overgrown field and told me that this was the actual location of the septic.  In other words, if the orange dye had surfaced, it would have shown up in that field.