There's a bunch of junk to the side of [Granary's second spawn point] ledge that [spies] can use to jump onto it.
Edit: Just looked and it's only true for blu's spawn. You can still get up to the boxes by taking the ramp in the garage.
I'm having trouble making heads or tails of this. Here's the relevant screenshot:
Sure, I can jump onto the tires and that might let me get a poorly placed teleporter, but that seems to be it. I also can't find the "garage ramp" Transall refers to.
Can a spy reach the other team's ledge?