Seems that what happened is that Soviet side had not informed the Finnish side about the Soviet car (two member of the Finnish delegation) arriving and coming over the border.Juha Tompuri wrote: ↑08 Dec 2019 21:11http://old.rk.karelia.ru/blog/trudnosti-perevoda-2/“The chief of the border troops of the Leningrad district reported that on October 15, 1939, at the site of the Sestroretsk Red Banner Border Detachment, the Finnish delegation headed by Paasikivi was supposed to go abroad by car. A bridge for auto-communication across the border river Sister is missing. In this regard, measures were taken to prepare a railway bridge in the vicinity of Art. Beloostrov.
Arriving at 11.30 on the border in the area of the bridge, a group of commanders of the headquarters of the troops and the border detachment found that the Finnish side was not ready to pass cars.
Major Comrade Okunevich, assistant to the USSR border commissar, tried to call Finnish border guards who were nearby, but the latter rushed to a firing point near the border and fired a machine gun in two shots at our commanders.
Our border commissioner on the Karelian Isthmus was invited to protest to the Finnish border commissar about the illegal actions of Finnish border guards.
From the report of the chief of the border troops of the Leningrad District on October 16, p. It is known that the Finnish command was not quite satisfied with its border guards guarding the railway bridge, which on October 15, when Soviet commanders appeared at the border in panic, fired at them from a machine gun.
Deputy People's Commissar of Internal Affairs of the USSR
The Soviet border guard unit had also started to carry planks to the railroad border bridge, to enable the car to cross it.
Soviet activity on the border bridge towards the Finnish side resulted the Finish border guards firing two warning shots.
I think I have seen somewhere a Soviet drawing about the issue, but can not find it at the moment.