The Secret Service would have tracked down and questioned John W. Hinckley Jr. last fall, and possibly put him under regular surveillance, if it had been informed of Hinckley's firearms arrest in Nashville last October, Secret Service Director H. Stuart Knight said yesterday.

In a detailed testimony on security measures at the scene of Monday's attack on President Reagan, Knight told a Senate subcommittee that because the Secret Service had no sense of any particular danger that day, it did not advise the president to wear a bullet-proof vest for his trip to the Washington Hilton Hotel.

Knight also revealed that another potential assassin, "well-known" to the Secret Service as a possible threat, was in the crowd of onlookers Monday close to Hinckley, who is charged with attempted assassination of the president.

This other person, Knight said, was the one some witnesses saw acting in a "herky-jerky" manner just before the shots rang out.

Knight said agents trained to spot suspicious people saw this other person, questioned him and ascertained that he posed no threat to Reagan.

Both Knight and his boss, Treasury Secretary Donald T. Regan, who testified before a House subcommittee yesterday, stated emphatically that their initial reviews of the incident revealed no clear security mistakes.

"I am confident," Regan said, ". . . the Secret Service was doing everything that would normally be done to protect a president."

Both men went out of their way to emphasize that the Secret Service and FBI now have what Reagan called "an excellent professional relationship."

In the past, feuds between the two agencies have been reported, and the animosity was reportedly strongest in the aftermath of the assassination of President Kennedy.

Accordingly, Knight and Regan declined to second-guess the FBI's failure to forward to the Secret Service a report of Hinckley's arrest at Nashville Airport last October -- on a day when then-president Carter was in that city -- on a charge of carrying three concealed handguns.

"In hindsight," Regan said, it would have been better if the FBI had done so. But Knight said he could understand why FBI officials might not have considered the arrest relevant to presidential protection.

The members of the Senate Appropriations Treasury subcommittee, who heard testimony from Knight and other Secret Service officials, praised the service's work at Monday's shooting. But several senators, particularly Paul D. Laxalt (R-Nev.), pressed for explanations of apparent security gaps.

Laxalt wanted to know why the president's limousine was not waiting directly outside when Reagan left the Washington Hilton Hotel. "It appeared to be parked maybe 8 or 10 feet further than it should have been," he said.

Knight replied that the car is always parked a distance from the exit at that hotel so it is positioned to drive straight out the driveway, at high speed if necessary.

Laxalt and Sen. James Abdnor (R-S.D.) asked several times why Hinckley and other onlookers were permitted to stand in a press area a few feet from Reagan's path to the car.

Knight and agent Jerry Parr, chief of the presidential protective detail, said there was no "designated press area" outside the Hilton that day. They said a press area is not usually needed there because a president leaving that hotel generally goes straight to his car and leaves.

"Mr. Hinckley was behind a rope which was a public area, not a designated press area," Knight said. He said cameramen who say they complained about interlopers among the working press must have talked to White House aides, because no Secret Service agents heard these complaints. w

The senators' chief line of questioning concerned the Secret Service's intelligence on potential assassins and Hinckley's absence from any of the agency's lists of people to be watched.

Knight said four general criteria make the service "interested" in someone -- threats against prominent persons, history of mental illness, access to weapons and propensity to violence.

If the Service had known of Hinckley's arrest, Knight said, "We would have as a minimum conducted an interview with the gentleman . . . and as a result, perhaps something more."

Knight declined to be specific about "something more" but said the term could include "observation." He said there are about 300 or 400 people "that we are interested in, on, say, a daily basis."

Knight said his employes would welcome the senators' praise for the service, "because whether it be an agent on duty or a clerk in a field office, they feel what happened Monday."