Biggus, quite. On one operation that you will be familiar with we had a number of submariners on board. They were foisted on us and not invited and OC OSS was furious when he found out but . . .
We observed a particular manoevre by our target followed by a particular acoustic event which then ceased and the target manoeuvre ceased too.
Our passengers told us exactly what we had heard, what the target did about it, and why it had happened. As you say, priceless.