The Veteran, Agent Rhodes, is in the Biker Hangout (level 1). He patrols the floor, though he often enters the Growing Room that’s full of drugs.
Here, you should be able to complete “Paging Dr. Greenthumb.” Take the emetic rat poison from the shelf inside the Growing Room (the shelf’s next to a couple of bikers).
Find the sprinkler unit outside the room and put some poison in it. If you use emetic rat poison, the other bikers will simply get sick. If you use lethal poison, those fellas will be dead, too (counting as non-target kills).
Activate the device to complete the tally. If you made Rhodes’ tummy hurt, he’ll go outside. You can safely eliminate him without anyone else noticing.
All right, we’re done with this dude. Given that we’re in the Biker Hangout still, then we could pay a visit to Agent Tremaine.