Many, many thanks for all suggestions. Most promising seem to me to be:
(1) Fell the pool’s vinyl bottom for soft spots. The leak will have
softened the underlying sand/soil. (If the pool stands on concrete, you are
out of luck.)
(2) Look for inflows of sand through leak.
(3) Cut a patch from a rubber party balloon & stretch it over a frame made
from an old coathanger. (Saran Wrap might work too.) Pass this over the
bottom of the pool. It will pucker over the leak.
(4) If an air hose can somehow be introduced UNDER the pool, blow some air
through & watch for uprising bubbles.
(5) Call the store you bought the pool from–they sometimes have an
electronic (?) leak-finder gizmo.
(6) “Take bright blue liquid laundry detergent, place it in a big syringe
(no needle, thank you, the kind you use to give medicine to a baby or dog)
and let some out along all the seems in the liner, around all the drains,
and around all the input pipes…”
Other techniques all involved emptying the pool, which my neigbor doesn’t
want to do.