By Thom Patterson and Tara Mulholland CNN

(CNN) -- United Airlines is investigating how a baggage handler became trapped in the cargo hold of a regional jetliner during an hourlong flight from Charlotte, North Carolina, to Washington's Dulles International Airport.

The man was found unharmed after United Airlines Flight 6060 landed at Dulles on New Year's Day, according to a statement from United. The airline said the flight was operated by Mesa Airlines but offered no other details.

"It surprises me how a person's co-workers could let that happen when they're supposed to be checking up on each other," said Paul Thompson, a longtime ramp agent at Denver International Airport, commenting in general about baggage handling operations.

"It is up to the gate crew at my airline to check inside the bin before the door is shut."

Typically, a three-person crew works together to load the baggage compartment, he said. Depending on how long the compartment is, one or two workers go inside to pack luggage at the same time.

Baggage handling is a tough job, he said, which can often leave workers exhausted.

"We crawl in there, and we're working on our knees, just stacking the bags as nicely and neatly and efficiently as possible," Thompson said. "Sometimes there's a lot of downtime, especially during cold or bad weather."

Workers often find themselves waiting for passenger bags to show up -- maybe from other flights -- creating a "10- or 15-minute break in there when you could fall asleep," Thompson said. "That's the mostly likely scenario that would result in a baggage handler being left in the baggage compartment -- them falling asleep.

"We're not supposed to do it, but people do it all the time," he said.

"I've seen it happen pretty frequently," Thompson said. "If you're going to be smart about it -- if you're going to choose to fall asleep inside the bin, you're going to be close to the door and not hiding back in the corner."

In 2015, a baggage handler fell asleep and became trapped inside an Alaska Airlines flight that had just taken off from Seattle-Tacoma International Airport. The man called 911 on his cell phone and banged on the plane's bulkheads, which luckily resulted in the aircraft returning to the airport after a 14-minute flight. Alaska Airlines banned the contract employee -- who was unharmed -- from working for the airline.

Falling asleep with travelers' bags isn't as difficult as you might think.

Entry-level airline baggage handlers often work multiple jobs or extra shifts to make a living, Thompson said. "If they're 18 or 19 and making, say 10 or 11 bucks an hour," they may be "just trying to get by" and "pay the bills."

Being trapped inside a baggage compartment during a flight is no picnic. The hold is pressurized. There is plenty of oxygen, but some airline cargo holds are not climate-controlled, Thompson said.

During Sunday's flight from Charlotte to Washington, the Embraer E170 reached an altitude of 27,000 feet, according to FlightAware, an airline tracking website.

On some planes, a handle on the inside of the baggage compartment door would allow it to be opened from the inside, Thompson said. But air pressurization inside would prevent anyone from opening a door after the plane has been pressurized.

Radio traffic revealed that workers initially treated the incident as a possible security issue, CNN affiliate WBTV-TV in Charlotte reported.

"We're going to work it as a security incident until we can get some confirmation to who he is even though he's in trade dress for a ramper in Charlotte. The flight crew doesn't remember seeing him or anything like that," WBTV quoted radio traffic.

An airline representative told CNN that a vendor for United employs the baggage handler.