Comment by nilslindemann

Your intuitution is right. At least in Germany it is not legal if not asked for permission first.

https://www.nilsbecker.de/rechtliche-grauzonen-fuer-ethische...

See also the German Criminal Code, starting with §202a "Data espionage":

https://www.gesetze-im-internet.de/englisch_stgb/englisch_st...