The smoke from the burning M35 can be seen a little, and the humvee is completely behind the wall on the right side, but it doesn't matter that you can't see it. It didn't matter even if I went behind the whole warehouse, the frame drop still happened if I looked that way.
Walking up to the M35 so that the humvee was behind me resulted in only a drop to 70.
EDIT:
It actually might be a bug with the map. The drop happens very precisely when you point the screen at the front entrance of the warehouse:
