How convenient that the van just happened to come along.
Yeah, I’m thinking inside job.
Someone had access to the van. They rigged it to have mechanical problems at the right point. Either a part that would reliably wear out in x miles or possibly a wireless remote. And then they followed the target in their white van.
“robbers then unloaded barrels filled with gold” $4M in gold wouldn’t even fill a 5 gal. bucket.
Inside job.