so with more info, yes seems like it would be a pumped septic field. I would expect whomever the local health board is that approves septic installations to have records of what was originally installed. could it have been a replacement and they didn't get it permitted and inspected? being on a lake, it would seem that a health board would be all over a septic installation so that it doesn't pollute. find where the septic tank is outside - if that is below the level of the house slab, then the pump is probably after the septic tank. if the septic tank sits higher than the house slab, then the pump has to be before it and would probably be in the house. septic inspections for house purchases are a typical thing - I would highly recommend one in the same vein as your typical home inspection. if nothing else, it will locate all these components.