Tripler Army Medical Center medically cleared and released Petty Officer 1st Class Russell Matthews on Tuesday night, Coast Guard spokesman Lt. Gene Maestas said.
When the 36-year-old went missing in October, he was in the process of being discharged from the Coast Guard for illegal use of marijuana, Maestas said.
He hasn't been arrested. But Maestas said Matthews' unauthorized absence for three months and the circumstances surrounding his disappearance make him a flight risk, so his commanding officer ordered him into pretrial confinement at the Naval Brig on Ford Island while the Coast Guard investigates his case.
Maestas said he doesn't know where Matthews has been or what he's been doing since he went missing in October. He said that would come out in the investigation.
"We want to know, too," Maestas said.
The Coast Guard will decide what step to take next after a legal review and an independent review are conducted over the next seven days, Maestas said.
Matthews is a rescue swimmer with 15 years of experience in the Coast Guard. He's been stationed at Air Station Barbers Point on Oahu for five years.
Honolulu police said Matthews returned home Sunday, but he was incoherent and taken to a hospital for observation. He later called his command.
Matthews' wife reported him missing in October.
Police later found the guardsman's car abandoned at Kaena Point, a remote area of Oahu.
At the time, police said Matthews was emotionally distraught, and his friends and family were concerned for his welfare.
Crews from the Coast Guard, Navy, state, county lifeguards and Honolulu fire and police departments together searched more than 10,000 nautical square miles - on land and at sea - for Matthews. The search was called off on Dec. 13.