Last evening while sorting through other "junk" I found the original site survey that was required for the bank mortgage when I built the house back in 1982. To my surprise, the surveyor put in the dimensions of the septic tank location as well as the distribution box location for the leach field. If your mortgage is still held locally, the bank might have the survey and it might show the location. I know that where I live in CT, if you apply for a building permit for any renovation, that the health department must sign off before the permit is issued and they won't do that until you give them the location of both the well and the septic tank. I believe that this is so they can update their records.