Whenever another creature you control leaves the battlefield, put a +1/+1 counter on Ninth Bridge Patrol.
If Ninth Bridge Patrol and another creature you control die simultaneously (perhaps because they were both attacking or blocking), Ninth Bridge Patrol won’t be on the battlefield as its triggered ability resolves. It can’t be saved by the +1/+1 counter that would have been put on it.
Ninth Bridge Patrol’s ability doesn’t care where the creature went or whether it’s a creature in its new zone. It may have died, been exiled, returned to your hand, and so on. It won’t trigger if an object remains on the battlefield but ceases to be a creature.