I would think that anyone working in a sewer inspection van would keep the door open because it is highly likely that sewer inspection vans smell like, well, sewer.
If the van is loaded with equipment, or even if it isn’t, theft and robbery are common in most of the US. You can’t leave a van door open and not be extremely vigilant.
One thing for certain: that is absolutely NOT a sewer inspection van. Seriously, you ever worked trades? It is way too clean on the interior and not fitted for working dirty jobs, to say nothing of the visible surveillance workstation.
While I can honestly believe both (it was a surveillance van vs it was a sewer maintenance company), do you think that the intimidation and surveillance of Snowden or Assange won’t last until the end of their lives?
A) I would question why they would do the effort of still doing surveillance on him
B) if they do, they are usually so smart to keep the door closed
C) like others have mentioned, sewer cleaning comes with a lot of tech (I assume remote controlled machines)