Merle Unger was 18 when they first put him in the old Chambersburg, Pa., jail. He climbed out almost every night to see his girlfriend and play bingo at the Catholic Church. So when the town built a new jail they put Unger in it. He was the first to escape. When they caught him, he escaped again. The people loved it, and for some he became a local hero.

Yesterday, Unger was being hunted again. He escaped Tuesday from the Patuxent Institution in Jessup, Md., and last night 50 state troopers and Howard County police, with tracker dogs and helicopters, were searching the area by Rte. 1 and Interstate 95 where he was last seen.

Unger and two other inmates drove out the prison's main gates in a dump truck. The prisoners had climbed over a fence from the recreation yard to the maintenance yard, prison director Dr. Norma B. Gluckstern said, held a homemade knife to a maintenance worker's throat and took the keys to the truck. They abandoned the truck between Rte. 1 and I-95.

But the game is serious now: 14 years after his first escapes, Unger is a convicted murderer serving a life sentence and considered dangerous. Police say he may be armed. The other two prisoners were captured Wednesday night and returned to Patuxent, but the search for Unger goes on.

It was different before the murder. Blake E. Martin, the Chambersburg public defender continually summoned to Unger's aid, had a T-shirt that said: "Merle, baby, where are you?" Unger was one of his first cases, and after 14 years of following his exploits he's planning to write a book about him.

Unger escaped from Chambersburg jail in May 1975 and was soon recaptured. Two hours after being convicted of the escape, in August, he fled again in a hail of gunfire.

Captured in Carlisle, Pa., he piled chairs and tables four stories high and climbed through a skylight in the town jail, said Martin. He headed for Hagerstown, and after robbing a grocery store, he shot a policeman. The T-shirts disappeared, said Martin. Robert Bumsted, a former Chambersburg reporter who covered the murder trial said "it all turned sour."

Unger fled again while waiting for the murder trial. His brother and his sister slipped a hacksaw blade to him, according to Martin, and he sawed through a metal plate and climbed into an airshaft. He was captured in Florida.

Unger has spent most of the last four years at the Maryland State Penitentiary in Baltimore, where escape attempts have been less successful.

He requested a transfer to Patuxent at the end of 1977, and a few weeks later a guard noticed that one of the bars in Unger's cell was partly sawed through. Unger claimed the bar was cut when he got there. Martin said, that he had been set up. He spent 15 days in seclusion and was sent back to Baltimore.

His lates escape attempt was in January, when he claimed a barbed wire fence at the Baltimore pentitentiary. Prison guards found him on the roof.

Unger had arrived at Patuxent a week ago for psychiatric testing.