Their mission is expected to end in three months and the entire search effort could be called off after that following a meeting of key countries Malaysia, China and Australia Friday.
"If it's not there, it means it's somewhere else," Fugro project director Paul Kennedy told Reuters.
While Kennedy does not exclude extreme possibilities that could have made the plane impossible to spot in the search zone, he and his team argue a more likely option is the plane glided down and crashed beyond the area originally marked out by calculations from satellite images.
"If it was manned, it could glide for a long way," Kennedy said. "You could glide it for further than our search area is, so I believe the logical conclusion will be well maybe that is the other scenario."
Doubts that the search teams are looking in the right place will likely fuel calls for all data to be made publicly available so that academics and rival companies can pursue an "open source" solution a collaborative public answer to the airline industry's greatest mystery.
Fugro's controlled glide hypothesis is also the first time officials have given some support to contested theories that someone was in control during the flight's final moments.