Solved. "Mysterious ways" indeed
The current playback window has in the 1st column smth that looks to me like a selection field.
There I found a couple of titles marked with a "White Exclamation Mark" sign in a black filled circle.
You can luckily sort this colum, there I found around 10 files.
Strange thing: all those files from a folder that is not in the list of folders anymore.
Even stranger, that it pops up from yesterday to today.
I remember, that I performed a Win10 fresh install around October this year.
And when I newly installed MusicBee then I entered only this directory to scan for Music: K:\<myname>\Eigene Music
The other files (test wav files for a particular D/A converter) were under K:\data\Software\driver\.....
Maybe some left-over entries from the past ..
What irritates me is, that this popped up now without having changed anything in that regards.
Neither I changed the folders which I scan, nor did I remove anything under K:\data\Softwar\driver.
I really do not know what happened. Thanks anyway for your kind help and time.