Two lost humpback whales seen just before sunset Tuesday nearing their ocean home after a two-week sojourn through inland waterways may have slipped back into the Pacific overnight.

Rescuers launched several boats in an effort to find the mother humpback and her calf Wednesday morning, but have not spotted the whales, said Bernadette Fees, deputy director of the California Department of Fish and Game.

"If they have gone out and made their way past the Golden Gate, they have done so quietly," Fees said.

• Click here to visit FOXNews.com's Natural Science Center.

The pair were last seen Tuesday night less than 10 miles from the Golden Gate after they passed under another busy bridge and entered San Francisco Bay.

The whales passed under the Richmond-San Rafael Bridge on Tuesday afternoon, the last bridge along the pair's route before reaching the Golden Gate.

If the humpbacks were able navigate south around Marin County's Tiburon Peninsula and nearby Angel Island, few obstacles would have been left on their route past Alcatraz to the Pacific Ocean.

Rescuers planned to rely on commercial vessels and Coast Guard patrols on regular duty to spot the pair, which have been easily sighted throughout their inland journey, if they were still hiding in the bay.

Authorities sent several boats west of the Golden Gate to try to confirm the whales had returned to the ocean, officials said.

"The assumption is if we have not sighted the mother and calf by late afternoon that they have made their way out to the Pacific," Fees said.

The duo was first spotted May 13 and got as far as 90 miles inland to the Port of Sacramento before turning around.

Biologists originally had planned to attach a satellite tracking tag to the mother humpback, but gusty winds and malfunctioning equipment stymied that effort.

Despite the apparently anticlimactic end to the humpbacks' saga, which has attracted thousands to Northern California waterfronts, biologists said the chance to closely observe the pair for so long was invaluable for science.

"While they may have gone on their way, we still have the benefit of all the information we haven't had access to before," Fees said. "If we learned anything about these two, it is that they will do what they do when they want to do it."

Ariadne Green, 57, of Vallejo, was one of many who came to catch what may have been a final glimpse of the pair Tuesday after traveling last week to Rio Vista, where the whales circled for a week before heading ocean-ward.

She described the humpbacks' inland visit as a "profound spiritual experience" but was equally grateful for their departure.

"They need to go home now because their health is in jeopardy," Green said. "It's good to know they're on their way back."

Biologists said the saltier water where the mother humpback whale and her calf had been swimming since leaving Rio Vista helped reverse some of the health problems caused by long exposure to fresh water.

Lesions that had formed on the humpbacks' skin over the weekend appeared to be sloughing off, Fees said. Scientists also reported that a coating of algae that was clinging to the mother farther upriver had fallen away.

Recent photographs showed that serious wounds suffered by both whales also had appeared to begin healing, said Rod McInnis, a spokesman for the National Oceanic and Atmospheric Administration.

Antibiotics were injected into the whales on Saturday to try to slow the damage from the gashes, likely from a boat's keel.

A convoy of boats had been escorting the pair to protect them from heavy ship traffic in the bay.