This morning travelling from neath toward resolven when in front of me was a john pearce wagon with the right hand side back door flapping in the wind. 
On a dual carrageway ,ok it was early about 6.00am but how the hell didn,t the driver notice !!! Must have been knackered .so i woke him up with my air horns and he pulled in the next layby 
That happened to me years ago coming through Airdrie the trailer was empty and over a bad bit o road the catch that swings over the handle to take the seal or padlock had moved back up and the handle jumped out of the lock position and the door was open luckily I caught site of it in the mirror after that I allways put a padlock on or a seal. Eddie.
Funny you should mention this, I was in Sarf London yesterday, stopped for a 15 somewhere round Woking, gets to my drop and the gateman asks if I knew my back door was open, WTF, It had the dog clip in when I set off in the morning and I hadn’t used the doors. I didn’t have any IBC’s missing though.
similar scenario I followed a trailer with flapping door along a66 to Keswick recently ,flashed driver and it transpired that he had stopped for a splash at services and someone had ,had themselves a look in the back . could have been nasty if it had “connected” with someone/thing
Used to be a regular trick in Liverpool especialy when the schools were on holiday. They would stand by pedestrian lights and then a little darling would press the button and you would wait and the first thing you knew about it was when another driver let you know that your rear doors were open, or you noticed it in your mirror. The mates of the one that crossed the road had opened your doors anything of use thrown out and you had blissfully carried on your way.