Health authorities are checking all venues visited by an Egyptian soldier and other members of his delegation during their stay in Rayong, after he tested positive for the coronavirus.
The visit has also revealed lax screening procedures at U-tapao airport in Rayong.
The visitors stayed here en route to a one-day trip to China, where they had unexplained military business, and again on the way back.
They were allowed to go outside the hotel where they were staying during their visit. The group did not need state quarantine because they were here less than 14 days, but they were required to undergo a virus test.
The places they went to included a shopping centre, which has not been identified.
Taweesilp Visanuyothin, the Centre for Covid-19 Situation Administration spokesman, said on Monday officials from Bangkok will join local staff in the eastern province in checking for coronavirus contagion at all places they went to, and people who were in close contact.
The health scare follows a positive result returned from the lab on Sunday after tests on a swab taken from the 43-year-old soldier. It showed he was infected with the Covid-19 virus.
He was one of the three new confirmed cases announced by the CCSA on Monday. The other two were Thais returning from working in Kuwait and Bahrain.
The soldier and 30 others, including crew of their aircraft, departed Egypt on July 6 and went to the United Arab Emirates. They went on to Pakistan the following day, Tuesday, and arrived at U-tapao airport in Rayong on Wednesday.
The UAE, Pakistan and Egypt are all hotspots for Covid-19 infections found in Thais returning home from overseas.
The information posted on the CCSA Facebook page said the group stayed at a hotel in the province.
The team left U-tapao for Chengdu, China, on Thursday. They returned shortly before midnight the same day, and went to stay at the same hotel.
Dr Taweesilp said health officials tested them on Friday, and there were unconfirmed signs that the soldier might have caught the virus.
The group returned to Egypt on Saturday. The test result confirming the infection came back from the lab on Sunday.
The spokesman said the hotel where he and other members of the delegation stayed was being cleaned . They visited other places during their stay in the province, including a shopping centre, he said. He would not name the places.
"We will examine all the places where they visited," the CCSA spokesman said.
Authorities would also improve the scanning process at the airport in Rayong, which was not as strictly enforced as it was at Suvarnabhumi airport, he said. "We will reconsider all practices," he said.
All were allowed to enter the country as they were defined as air crew under the conditions set by the Civil Aviation Authority of Thailand, according to the centre.
It remained unclear why they were allowed to go outside the hotel during their stay in Rayong. The CCSA did not go into details how many of the group were soldiers or civilians.