Rusty arrived at the zoo in April from a zoo in Lincoln, Neb., and was in quarantine for several weeks until he went on exhibit in early June. He will turn 1 year old in July.
Red pandas are highly territorial, so zoo officials did not believe he would have traveled far. Rusty, it seems, wanted to explore his new city.
Animal keepers discovered he was missing Monday morning and started searching at 8 a.m. The zoo began sending out messages about his disappearance Monday morning on Twitter and Facebook in case someone saw him.
A spokeswoman said the zoo was "incredibly grateful" to Ashley Foughty who lives nearby, saw Rusty, tweeted a picture and called the zoo. She apparently had to leave town on a trip Monday, so zoo officials couldn't thank her in person.
Zoo Director Dennis Kelly said officials will thoroughly review the incident and said it's rare for any animal to escape.
"We will not let this happen again," he said. "Before we put Rusty back, we'll go back over this exhibit with a fine tooth comb."
Animal escapes are very rare among accredited zoos, said Steve Feldman of the Association of Zoos and Aquariums. They have primary and secondary containment systems for animals. The most frequent escape is when a bird flies away.
National Zoo officials could not recall another such escape in Washington in recent decades. In 1983, a teenager was bitten by one of two viper snakes he was suspected of stealing from the zoo's reptile house. The boy carried away the snakes in a plastic garbage bag on a city bus after hiding in the zoo after it closed, officials said.
The female red panda, Shama, remained on view in the leafy exhibit Monday, despite the hoopla over her mate.