Here are the ones I can think of:

-Joe civ with concealed carry
-Joe civ using vehicle as a weapon to get a mobility kill by ramming the getaway vehicle after observing shot.
Joe civ with cell phone observing&reporting an event
-car accident (oops!)
-Something transparent to the sniper that he/she has not factored in to their risk assessment (besides what has been listed already).

I believe these guys are comms cold during infil/exfil, but are continually, passively monitoring LEA freqs.  They may have a security element that has not revealed itself yet.

The LEAs need to wait until daybreak to scour the crime scene.  They are ruining whatever clues may be there by stomping all over it tonight.

I'm guessing a truck driver will spot the vehicle in a rest stop with the killers asleep inside.

Just a gut feel!