Always did
Right now, Russia doesn't have all that much military capability near St. Petersburg.[2] Some units are off at the Ukraine war. The Baltic Fleet is weaker than ever.
There's a "Free Ingria" movement that wants to make all of Leningrad Oblast a separate country. It's not going anywhere without outside support. It might get some.
[1] https://apnews.com/article/finland-estonia-cable-undersea-ru...
Closer to 90km. If it was 20, all ships to/from St Petersburg would have to pass through Estonian/Finnish territorial waters and we wouldn’t have questions around jurisdiction.
In terms of potential hybrid warfare targets, Kaliningrad looks soft. Many of the residents have little loyalty to Russia. A partial blockade combined with some sabotage and support of separatist organizations could accomplish a lot.
It's a narrow corridor to patrol. Small boats watching large ships can tell if an anchor is run out. An ordinary fish-finder sonar should be able to tell if a ship is dragging something. Finland has many small coast guard vessels, and a long history of pushing back against border incursions from Russia.
The most recent incursion was [1]. "Since the beginning of August 2023, more than 1,300 third-country nationals have arrived in Finland from Russia without a visa. According to the authorities, it was clear that foreign authorities or other actors were facilitating instrumentalised migration. This phenomenon and the risk of its escalation posed a serious threat to national security and public order in Finland." Migrants who escaped Afghanistan, etc. and ended up in Russia are apparently being given a hard choice - join the army and fight in Ukraine, or leave Russia.
So Finland closed the land border with Russia completely. Russia objected. Finland did not give in. Finland started building a border fence on the Russian border, with surveillance gear, back in 2023.[2]
[1] https://valtioneuvosto.fi/en/situation-at-finlands-eastern-b...
[2] https://www.globalconstructionreview.com/finland-starts-erec...