I grew up on an island in the Baltic. Cable lines to main land snapped by anchor dragging was a surprisingly common occurrence. Other marine shenanigans included drunken sailors beaching their ships because they forgot to turn around the island. And ofcause a ton of illegal fishing.

Your conclusion seems most likely in my book.

Agreed. Bottom trawling was the first thing I thought of. The machinery is probably relatively nearby, possibly with shreds of net caught in it, sitting on the bottom.

How big are these sensor objects? How do they communicate what datat they collect?

Presumably by the cable that was torn off.

"No boats - not even local fishing boats - are allowed into the area, called Eckernförde Bay, about 70km (44 miles) south of the Danish border."

Seems like a risky place to go trawling.

A comment in this thread shows the area. It's pretty small and would be very easy for someone to drive through without realizing it.

Two units connected by a cable, weighing a combined 1700 pounds, both dragged away by accident? I have a hard time imagining that. In an accident where something on the surface snags it, I can see one of the units getting pulled away and the cable snapping, but both seems unlikely, at least for an anchor. Granted, I don't know much about what a trawl net looks like in the water or how much weight it can hold.

why don't they put accoustic retroreflectors (perhaps with a tailored high q resonant frequency response on it) on the thing?

then its just a question of raising the signal from the noise floor at different locations to locate it...

This is my bet as well.