Continuity mistake: When Dom's car crashes against Hernán and Zizi's, there are two oil spills on the road. Dom stands up, and the oil spills are gone; plus, the sunlight on the pavement has changed. The position of the line attached to Dom's car also changes several times between shots.
Continuity mistake: When Zizi dies, his arm lies on top of the line. A shot later, his body has moved half a metre away.
Continuity mistake: When Brian saves Dom on the bridge, Dom's T-shirt swaps from straight to wrinkled between frames, and the sweat stain disappears.
Answer: Hobbs doesn't see that scene, though he probably does guess part of what happened - ie. that they swapped the vaults. The scene is played as a reveal for us, the audience, to show us how the vault that is supposed to be full of money is now suddenly empty.
roboc