A witness on board the plane said the crew emphasized the object could not be identified and it was not known if it was related to the ARA San Juan.
The lives of the 44 crew members hang in the balance a week after the submarine disappearance following a technical fault.
The plane, a P-8A Poseidon, returned to its base in Bahia Blanca, Argentina late on Wednesday.
More than 4,000 personnel from 12 countries are now scouring the South Atlantic search zone, braving stormy conditions with high winds and waves up to seven metres high.
Bad weather conditions on the sea making waves like “a rollercoaster” are hampering the rescue efforts, but there are ships specializing in the rescue of submarines that are searching the area”.
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Britain’s HMS Protector is already helping the efforts, with HMS Clyde expected to reach the search area on Tuesday.
Russia’s defence ministry has sent an oceanographic research ship to the South Atlantic to help with the search for an Argentine submarine which went missing more than a week ago, reports TASS news agency.
The vessel, called Yantar (Amber), is equipped with two self-propelled deep submergence vehicles allowing it to examine underwater areas up to 6,000 metres (3.75 miles) deep.
Relatives of the crew members have been gathered at a naval base in Mar del Plata, where the search is being coordinated. Among the missing crew is Argentina’s first female submarine officer Eliana Krawczyk, 35, and Luis Niz, 25, who is due to get married in two weeks’ time.
Elena Alfaro, whose brother Cristian Ibanez is serving as a radar expert on the San Juan, told Reuters: “We came today because we had hope that they had returned.
“It is incomprehensible that so much time has passed. We are in pain.”
Around 30 boats and planes and 4,000 people from Argentina, the United States, Britain, Chile and Brazil have joined the search for the submarine, which last transmitted its location about 300 miles out to sea.
Shortly before making its last contact, the submarine’s captain reported it had experienced a “short circuit,” and was told to change course and return to base.
The news comes after it was revealed that the Argentine Navy believe the sub has enough oxygen to last until Tuesday amid fears the vessel had run out.
Captain and commander of the submarines in the Spanish Navy, Alejandro Cuerda Lorenzo, told radio show ‘Herrera en COPE’ the Argentine navy considers “that the survival on board could be until 28 November”.
He added: “I am not pessimistic or optimistic, I am enthusiastic.”
The Captain says calculations from the Argentine Navy indicate the oxygen reserves of the submersible would reach the limit on Tuesday, giving them five more days.