Under the searing sunlight, hundreds of customers lined up outside an In-N-Out pop-up store that exclusively opened up for just four hours in Gangnam-gu, southern Seoul, Wednesday.

"People have been lining up since 1 a.m.," said a part-time worker at the In-N-Out pop-up store.

"We sold out even before we opened -- all the 500 wristbands that are needed for an entry into the restaurant have been given out to the people standing in lines, before 11 a.m.," he added.

In-N-Out, often considered on a similar level to Five Guys and Shake Shack among US burger franchises, launched a pop-up store at Centre Cheongdam from 11 a.m. to 3 p.m.

This year's pop-up event, which is directly organized by the chain's US headquarters, marked the brand's fourth pop-up event in Seoul since 2012.

The pop-up store exclusively sold 500 burgers from In-N-Out, including a Double Double Burger, Animal Style Burger, and Protein Style Burger on a first-come, first-served basis.

According to an In-N-Out official, the one-day promotional event by the US burger chain came as a part of the chain's US headquarter's initiative to advertise its goods in overseas markets.

"This is a part of our global marketing strategies. We do pop-ups in multiple countries, and we give some of (the franchise's products) out (to global consumers) to (have them) try our burgers. We do various countries throughout the year, and we're happy to be back in Seoul since 2019," Luis Hernandez, manager of special foreign events at In-N-Out told The Korea Herald.

"We expect the prepared goods to be gone by 2 p.m.," he added.

Since first opening a pop-up store in Gangnam-gu, Seoul on March, 2012, the US burger chain also opened pop-up stores in Korea on April, 2016 and May, 2019, respectively.

Despite these ostensibly promotional events, the burger chain has yet to officially announce its entry into the Korean market.

Industry watchers say the real motivation for the pop-up events is extending the brand's trademark rights in Korea. According to Korean law, a trademark right is open to cancellation if the registrant of the trademark does not conduct business with the trademark for more than three years.

They added that the brand's efforts to extend its trademark rights here might hint at plans to foray into the Korean market soon.

"A globally famous brand does not generally need to register its trademark (in a foreign country) to protect its intellectual property rights, because if a new, overseas brand attempts to use its logo, to say, 'borrow its brand power', it can sue them on the basis of the Unfair Competition Prevention and Trade Secret Protection Act," a patent attorney working at Chunji Law told The Korea Herald.

According to him, although In-N-Out's true purpose is unclear, the fact that the brand is putting in periodical efforts to extend its trademark rights in Korea could be a sign that the brand is positively reviewing the launch of its Korean operations.

An insider at a local food company added that it was likely that In-N-Out is looking to enter the Korean market.

"A trademark registration in a foreign country doesn't always mean that they are looking to launch business here, but the possibility is high that they are currently looking over the plans," he said.

"However, as In-N-Out has strict in-house policy to prioritize the quality of its goods, the process for its overseas launch is expected be difficult," he added.

Amid the burgeoning market for hamburgers here -- which more than doubled in a decade to 4 trillion won ($3.02 billion) last year -- a slew of global burger chains have started their Korean operations.

In 2016, Shake Shack Burger was introduced to Korea by food giant SPC, with chicken franchise BHC also opening Korea's first outlet of US fast food chain Super Duper Burgers in Gangnam, Seoul, last year.

US burger chain Five Guys, run locally by Hanwha Galleria, is also set to open its first store in Korea -- making Korea the fifth country in Asia for the burger franchise to open outlets.