When you enter, you're required to fill out a health declaration form, which asks for your contact info. IIRC, it's not cross-referenced with immigration database. Naturally, some were not filled out correctly, maliciously or not.

Of the reported cases, police got involved to track down said persons, and they were fined depending on the severity of the infraction. I don't know if the form DB is now cross-checked with the immigration DB.