A feacal deposit left in the toilet bowl following an unsuccessful flush. Due to the density of human excreta these visual obscenities usually reside at periscope depth. Simple post-flush checks by the degenerates that leave them behind would render the Brown Submarine a thing of the past.
Bridge Officer: Captain! Bearing 030 off the starboard bow. Brown Submarine!
Captain: Set Depth Charges to Zero...Fire 1...Fire 2...
Bridge Officer: ....Enemy flushed sir.
Captain: Good work sailor, the Rear Admiral will be pleased.