Investigators used GPS to track a lorry driver in the hours before a foiled attempt to smuggle migrants across the Channel, a court heard.
An "orchestrated" Eurotunnel check led to the discovery of 20 Vietnamese migrants at Coquelles, near Calais, on 14 October last year.
Prosecutors said some of them were among the 39 migrants found dead in a lorry in Purfleet, Essex, days later.
Four men are on trial at the Old Bailey in connection with the deaths.
The prosecution claimed the people smugglers made a fatal decision to double up their human cargo after an attempt was foiled by French authorities on 14 October.
Haulier Christopher Kennedy, who was in northern France, received nine texts that day coinciding with the time messages were sent by boss Ronan Hughes in the Republic of Ireland, jurors heard.
At 23:50 BST, Mr Kennedy was stopped by officers at Coquelles and the 20 Vietnamese nationals were found in the back of his trailer, the court heard.
Prosecutor Jonathan Polnay read a statement by French officer Maxime Saison, who was on duty in Coquelles that night.
The officer said he was "advised by our local management of an orchestrated check testing positive on a heavy goods vehicle based at the Boulevard des Flandres at Coquelles which gave officers to believe there were individuals inside the trailer.
"These officers took us to see 20 people found hidden in the Bulgarian heavy goods vehicle … driven by Christopher Kennedy."
The migrants confirmed in English they were Vietnamese nationals but had no documentation with them, according to the officer.
Mr Saison said the migrants were frisked and taken away without restraint.
Mr Kennedy, 24, from County Armagh, is not accused of being involved in the fatal transportation of migrants on 23 October.
He denies being part of a people-smuggling conspiracy with fellow haulier Eamonn Harrison, 23, of Mayobridge, Co Down, and Valentin Calota, 37, of Birmingham.
Alleged key organiser Gheorghe Nica, 43, of Basildon, Essex, has admitted the conspiracy but denies 39 counts of manslaughter with Mr Harrison.
The trial continues.