Well this is one of the things which no-one seems to be able to remember about it... does the tank outflow go into an infiltration system in the field downhill from it (which, on the face of it, looks perfect for such a thing), or does it actually just go into a convenient ditch on the other side of a nearby wall? (Well defended by brambles so it's not anywhere anyone's gone looking in recently.)
Noone's been able to locate any old paperwork from the installation, and no-one remembers it ever having had any maintenance or pump-outs. Over multiple decades! But then the place has plenty of extended empty periods.
Travel restrictions permitting, someone will hopefully get a last visit in before year end to drain the water system before winter, and be able to have a poke around down by the tank now we have a better idea what we need to look for. I half wonder if we should try and do a "dye test" but not sure how much water you realistically need to put through to have a chance of seeing anything.