Assuming you don't have LinkedIn or any social network that says who your current employer is, can your previous employer somehow find out through a certain source or access this information from somewhere to learn who you are currently employed with? Or how.
For example, there are agencies in Germany such as Job-Center, Agentur für Arbeit, or immigration-office that normally have such information, but I wonder if they would disclose such information given your previous employer's query from them.
In general, how easy it can be for a previous employer to access such knowledge about you in Germany.