The group were freed from the vehicle in Germany yesterday.
Seven adults and a child needed hospital treatment after being found by police at a service station on the A3 autobahn near Cologne.
Motorists using the rest stop at Königsforst heard loud banging on the side of the truck and informed the driver who in turn contacted police.
All were suffering from the first stages of hypothermia.
It remains unclear how the migrants – understood to be from Iraq – came to be in the refrigerated transporter but police estimate they had been inside for four hours.
They either climbed aboard, or were secreted aboard by people smugglers – in Belgium and told police they thought they were going to the UK; but the truck was in fact heading for southern Germany.
The Polish driver is being held for questioning.