As this was going on, the NSA satellite KH-14 was making a pass over the Soviet Union. It saw the airfield used by the 76th Guard Airborne Division preparing for a massive operation. This important moved at speed not normally seen by intelligence agency, and was bring shown to President Reagan only 3 hours later. This information was them passed on to Sweden 30 minutes after that.
The race was on.
That early warning will prove invaluable. Sweden is lucky the US and the USSR don't like each other. While I'm sure they could have guessed at an invasion, knowing for sure (and possibly knowing force sizes) could be a game changer down the road.