He was arrested because he posted a bullshit reply to one of Greta’s posts, in that reply he had pizza boxes of a local Romanian pizza place. Someone publicly retweeted to the local Romanian police that he was in their area.

Sure, Romanian authorities knew he was in the country, but it was only the local tip off (and the general international publicity) that led to his arrest and prosecution. The Romanian authorities couldn’t not act upon that publicly displayed information.