Acoustic tags that transmit pings from 30 khz to 50 khz are used to track various species of large marine animals (i.e., sharks, whales, sea turtles, tuna, seals, etc).
And these tags are mobile. If the carrier of the tag was responsible for each located ping being in a different area then it is probable that at least two pings would have been on different, Doppler-shifted, frequencies to account for the variation in locations.
Had there been such a tag set to operate at that frequency I would have expected to someone to come up and say, 'Hey guys, you are tracking my great white . . . '