https://www.mhlw.go.jp/stf/newpage_09312.html (19th)

Overview
(1) Age: 50's
(2) Gender: Male
(3) Place of residence: People's Republic of China (Hubei)
(4) Symptoms, progress:
He came to Japan on January 22.
No symptoms until January 25.
Fever from January 26 morning. It is 37.5 degrees at 12:00. Transported to a medical institution by ambulance at night. Pulmonary inflammation was observed.
On the same day, a specimen (pharyngeal swab) was collected based on surveillance of sickness.
On January 27, the test was found to be negative.
On January 28, fever was released. Pharyngeal swab, sputum collection.
Returned to China on January 30.
The sputum and pharyngeal swabs stored on February 3 (collected on January 28) were inspected and sent to the National Institute of Infectious Diseases.
On February 4, the test was found to be positive.
(5) Action history: The
history of exposure in Wuhan city is unknown, but participated in the same bus tour as the fourth confirmed case in Japan (confirmed on January 26, male in his forties).
Wear a mask after onset.

Quote

On January 27, the test was found to be negative.
On January 28, fever was released. Pharyngeal swab, sputum collection.
Returned to China on January 30.
The sputum and pharyngeal swabs stored on February 3 (collected on January 28) were inspected and sent to the National Institute of Infectious Diseases.
On February 4, the test was found to be positive.

Does this mean that the first test taken was done wrong or was somehow 'faulty' or that the virus was undetectable until a later date?

@psk

Yes it does seems so. Thank for pointing this out. I missed that detail.

It happened once in Canada  also.

You're welcome.

 

That's quite alarming.