The peppermint test is one that we old time plumbers have used to find leaks in plumbing waste and vent lines. Absolutely the safest test you can do. I use cooking peppermint. You can buy it in the grocery store. It has to be done carefully, because if you spill it in the house everything smells like peppermint and you can't find the real leak. Always soak your rag outside (the best is up on the roof), try not to get it on your hands. Then put the rag attached to a test gauge cap on the end of the pipe so you can pressurize the system with no more than 1/4 ounce of pressure. If you use more than 1/4 oz of pressure, it will blow the traps and simply come out in the rooms without finding the leak. Make sure you put a rubber plug down into the main sewer to close it off so the waste and vent system is a closed system. If you do this, it will take about 1/2 hour to find the leak if it is in the wall, if its in the open, you will find it pretty fast. Just follow your nose.