I am not sure that you want to make this an issue for your customer. It is your issue.
This problem is actually at least 10 years old. At that stage we also had many thousands of migrants in Calais and breaking into trucks to get into the UK was the favorite thing to do. Especially at all the parkings around Calais where truckers were having their coffee of stopping for the night.
Food products might no longer be food safe once they have had the risk to be "tampered" with.
A lot of the risk factors were already mentioned above.
I have seen a few "tampered" trucks 10 years ago and these became insurance claims (at a minimum with partial write offs, or full write offs)
These were chilled products and we could show that the cooling units had been blocked (and the temperature graphs showed this as well). Also we could find evidence of wet pallets, likely to be urine. some product has also been opened and consumed. These trucks were diverted to an area where they could be inspected and handled with an insurance agent and a solution could be worked out.
Also think of food defense.
Make sure that you not only have numbered/unique seals on the lorry/truck, but also that they are properly locked, as a prevention that migrants can get access to your truck. Before entering and during stay in the port loading area make sure you check your seal and lock. (the port authority might also scan you with Infrared). If you find an issue, report immediately and wait for instruction before continuing the journey.
The UK has this leaflet: https://www.gov.uk/g...rry-leaflet.pdf or the older version that was maybe better http://www.trucker.d...t June 2010.pdf. Both have good advice that your truckers should be trained on and made vigilant.
https://www.gov.uk/g...urity-checklist (also for your truck drivers in different languages)
Example of lock: http://www.transport...uard-locks.html (also helps to let them look for another truck instead of yours...)
Keep Food Safe!