how did they manage to bring themselves and some $13 billion worth of gold past the US-Canadian border unnoticed?
It's never explained, but we can take two things into account:
- the line between the 2 countries isn't that sealed a border.
- the aspirin bottle comes from the Canadian part of the border.
First, as a foreigner, when living in the US, I was able to cross the Canadian/US border twice in a day without my passport (I had forgotten it at the hotel). It was 1992. It seems to be much harder nowadays.
Second, the "Germans" probably came to the US from Canada, where they bought the aspirin. They chose a place where trucks going back and forth are so common that controls are less than ideal, more chances to go unoticed. It's not the last stop at the border going to Tijuana.
So it seems pretty easy for the terrorists to have customs papers ready just in case. Trucks have been stolen a while ago, giving time for some proper preparation. So people working and their trucks, in an area full of trucks. If you want to hide something, just put it where it's natural for it to be.