Sometimes, a lump of ice forms on the outlet. If that lump breaks free, then it can have a radar return like the one described.

From STS-41:

As a result of the data review of the supply water dump at approximately 68 hours MET (mission elapsed time), it was determined that ice had formed around the supply dump nozzle. There also were indications that ice had formed around the waste water dump nozzle, but it was believed that the ice did not remain. The RMS (remote manipulator system) was deployed dnd the RMS TV showed a large column of ice over the supply nozzle. A waste dump was attempted with TV coverage; ice buildup was observed and the waste dump was terminated.

and from STS-64

A supply water dump was initiated at 192:07:27:20 G.m.t. (02:14:44:20 MET). At 192:07:59:02 G.m.t. (02:15:16:02 MET) (32 minutes later), the dump was prematurely terminated because of excessively low dump-nozzle temperatures. Data analysis concluded that the supply water dump nozzle had accumulated an unknown amount of ice that may also have obstructed the waste water dump nozzle. The supply and waste water dump nozzle heaters were cycled to eliminate the ice from the nozzle area. After the supply and waste water dump nozzle temperatures indicated ice was no longer present, a successful waste water dump was initiated at 193: 16:49 G.m.t. (04:00:06 MET). The dump was completed 56 minutes later with good nozzle temperatures and no signs of ice. Supply water dumps were performed using the flash evaporator system (FES) for the remainder of the flight.