From a 2011 BBC article based on a Russian (Martirosyan) historian's assertion/research (the 18th of June is mentioned here, though may or may not be related to the parachute drop you mentioned, but I suspect it is in fact the one because by the sentences' context it may have been in Belarusia):
"Ordinary Soviet border guards passed on most of the warnings of the coming invasion", Martirosyan said.
Between 1 and 10 June, they captured 108 enemy spies and saboteurs, he told Komsomolskaya Pravda, and a further 200 or so in the final 12 days before the invasion.
On 14 June, guards on the Belarusian section of the border relayed back to Moscow the correct date of the planned invasion, learnt from two captured saboteurs. The same date was revealed by saboteurs captured on 18 June.
Border guard agents operating on the German-controlled side of the border also confirmed the date repeatedly, Martirosyan found, as did local civilians.
Most poignantly, perhaps, were the Polish women who gathered on the opposite bank of one frontier river on 15 June, cupping their hands around their mouths to shout warnings, in broken Russian, to the Soviet guards facing them.
"Soviets, Soviets, the war is coming!" they were recorded as saying. "Soviets, the war will start in one week!"
Only on 18 June did Stalin order aerial reconnaissance missions to be conducted along the USSR's western borders.
Flying 400km (250 miles) from south to north, one pilot, Air Maj-Gen Georgy Zakharov, reported seeing "frontier regions west of the state border packed with troops... tanks, armoured cars and guns poorly concealed or not concealed at all... roads criss-crossed by motorcycles and what appeared to be staff cars". (end of BBC article excerpt).
I note that the nationalities of the "spies and saboteurs" in the article is ambiguous- ergo, not necessarily German.To me, this brings home the reality that it wasn't just Brandenburgers involved,they most likely had some Belarusian, Ukrainian, Estonian/Latvian/Lithuanian/ Romanian nationals (backed by the Germans) going across and assisting in creating disruption sometime before (and during) the actual invasion. If you believe that 300 spies and saboteurs were caught by the Soviets in the 3 weeks before the invasion, then there had to be more than that sent over, I would presume.( I would also be interested in seeing the Russian historian's research methodology).
That seems pretty rampant/blatant even given the scale of the invasion front. And combined with the frequent German pre-Barbarossa aerial reconnaissance being flown the German pre-war boldness can be perceived as bordering on the absurd. (Of course that is another story).
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