dpb wrote:

How do you know it was "all"??? That would take a volumetric
measurement on both sides which undoubtedly wasn't done. Not that I
would suspect too much, just a point that you can't draw excessive
conclusions from simple observations.

As for the overall situation --

First is the question of the tank design, etc., ... was/is there a
siphon that could account for the permanent level?

Were any cracks/potential leaks that were fixed/repaired below the
level that could account for it? I've not read the whole thread by
any stretch but did see indication that what appeared to be most
problem(s) may have been at the level of the outlet which wouldn't
have explained the low level anyway. If misread that or that's
wrong, ignore...
Is the house presently occupied? If so, it shouldn't take any time at
all for the level to reach the outlet level (in overall terms); normal
household activities will put quite a level of water down the drain
in a day or so unless there's some very stringent usage restrictions
being observed by the occupants. If so, observing the level should
lead to fairly quick conclusion as to whether there's an escape path
somewhere or not.

If it's unoccupied, and the tank is still open, I'd just fill it up w/
hose and other running water until it is at the outlet then watch to
see if it goes down from there for a period.

As for leak in exit line from house, how deep is that line and what's
it made of? Unless it's quite deep it shouldn't take much
investigation to discover (particularly since you say it's short)
whether there's waterlogged soil in the area -- even a probe rod
should tell you the answer of whether there's saturated soil or not
close by.
That last holds for areas around the tank as well--if you can probe
deeply somewhere indicating soft ground, that would be telling of
leaking--of course if it's in area of repair above, that could/would
likely be old, not new.

All in all, if the house has been occupied and used in normal fashion
w/ reasonable number of occupants and there are no visible
indications I'd tend to think it's not a major problem.

You might want to write in a clause in any contract that owing to the
circumstances any subsequent problems uncovered or becoming apparent
within oh, say one year, are to be covered 50:50 or something similar
by you and seller.

The only possibility that makes sense and conforms with the existing
evidence is that someone is stealing the fluids from the tank. Maybe at
night.

Be vigilant and maybe the answer will be discovered.