https://en.wikipedia.org/wiki/Russian_shadow_fleet

Various tactics are adopted to make it harder to track vessels in the shadow fleet; these include renaming the ships, changing their country of registration, sailing under false flags, switching off tracking signals, and broadcasting false locations.

Even disregarding all the politics around sanction evasion and espionage, this stuff is reason enough to seize these vessels.

Hilariously, this threat is actually a threat to send nuclear reactors into their regions and sink them because the Russian navy wouldn't pose a military obstacle to Belgium, let alone the rest of the EU.