The very first reports were that the truck entered at Holyhead, commenting on that I thought the driver was not involved, he would never have travelled to Essex if he knew he had passengers.
The driver would know what he was supposed to be carrying and he would know if it was an empty trailer, he would feel the weight difference when driving. Removing the seals is not something a driver would normally do, nor would he stop on an industrial estate 5 miles from the port. I’m not surprised the police are holding him, he has a lot of explaining to do.
With a driver cooperating it is not difficult to get into the UK, I have driven to France many times with a caravan and could easily have had a stowaway but I’ve never been checked