So what's the solution ? Assign a surveillance UAV to every Russian ship parked "without a good reason" over a cable ? It would be expensive, but doable, and create a reserve of vehicles for wartime use.
The solution is to refuse passage of the ships to\into Russian part of the Baltic. Naval blockade. But it is impossible as so many Russian-backed politicians are elected in EU. And all other doesn't have a strong will or doesn't care about Russian threat. No Churchill\Reagan types are there.