What reason would they have for not wanting it found?
To make sure family picked up the life assurance payment. Often these exclude suicide, especially (IME) if you do it within 1 year of starting the policy.
IIRC the underwater pings were being picked up, at one stage, for a couple of hours, and that must exclude any possibility of a spurious signal, surely? There would also be a gradual change in frequency (unless the source is crystal derived) which the manufacturer could reproduce with end of life batteries in a test unit. And if the frequency is crystal controlled then it would be ultra stable long-term which would be hugely obvious over 2 hours (a few ppm long term variation) and the approx 7000 pings collected.