TOKYO -- Signs of a hole in Japan's measures to lock down its borders against the coronavirus have been spotted in an unexpected place: a department store in downtown Tokyo's ritzy Ginza district.

An employee involved in analyzing customer data there began feeling a creeping sense of unease last fall. While looking at passport information from shoppers taking advantage of tax exemptions for non-residents, he noticed that some had come to the store not long after arriving in the country.

It was clear that they had breached a two-week quarantine requested by the government.

The pandemic caused foreign travel to Japan to plunge more than 90%, but not to zero. A trickle of visitors has continued to flow across the border. The government asks everyone entering the country, regardless of nationality, to stay at home or in a hotel for two weeks after arrival, and to report their location and health status regularly via a smartphone app.

But these conditions are not legally binding. The health ministry says it does not hear from as many as 100 of the 20,000-plus people who need to check in each day. Some likely go out shopping regardless of the government's demands to self-isolate.

Before the pandemic, many foreign visitors took advantage of Japan's tax-free shopping program.

Information on department store databases can pinpoint which customers may have broken quarantine because they need to show the entry date stamped on their passports to receive sales tax refunds.

When the employee in Ginza combed through his store's data and confirmed his suspicions, he wondered at the same time whether the government knew how valuable this information would be.

Japan switched to digital records of tax-free shopping transactions in April 2020. This data is sent to the National Tax Agency, meaning that the government should have the same information that stores do.

However, officials face hurdles when trying to use this resource directly.

"The people who handle coronavirus countermeasures probably don't even know that the data exists," said an employee at an economic agency. Even if they do, there are legal restrictions on using tax data for non-tax-related purposes.

Visas and other essential passport information fall under the jurisdiction of the Immigration Services Agency, which faces separate barriers to using that data for border control measures.