I had a similar, and also first time, experience earlier this week as well on a LH Bos-muc flight. Soon after take off, the “ding” that sounds for the seat belt light started going off with 3 dings in a few seconds and then would repeat maybe 10 sec later. After the first few bursts the flight attendants start all moving aft with purpose but nothing was said.
There were quite a few glances going on around the C cabin trying to figure out what was going on. A few minutes later the pursuer came on the PA and gave a rather stern lecture about not smoking or vaping in the bathroom in both German and English.
Upon atrival into muc there was a group of German police waiting on the jet bridge looking very unamused.