If your current quest objective is to follow dogmeat, and you have the quest marked in your Pip-Boy, then you should have a quest marker leading to him. If that objective is complete, then you simply need to enter Kellog's base to continue. If it's not complete, but there's no marker, here are some things you can try:
I don't recall any law enforcement robots on the trail, but there is an assaultron shortly after passing Forest Grove Marsh. There is a cigar on the box the assaultron's wreck is lying upon, which is the clue here. You're supposed to grab it and show it to Dogmeat. If you did this, then he would have started toward the next clue. It doesn't seem like you always need to actually kneel down and show him the clue. Sometimes he just takes off.
If this is where you are, then the next clue is a bloody rag on a chain link fence up the hill toward the Greater Mass Blood Clinic. After the robot, continue up the road a ways until you get to the intersection. Take a right, then veer left up the hill.
The rag is next to a break in the fence that you can walk through. Maybe Dogmeat is over there. That's the last clue before Kellog's base
at Fort Hagen.
Entering the Base
If you can't find Dogmeat, that might be okay. I've previously ditched Dogmeat and ran ahead to enter the base, and everything went fine. I can't recall if that was definitely before the last bloody rag, though.
To do this, you need to enter
Fort Hagen, and get to the basement via the elevator after the ceiling turret. This elevator will be inaccessible before a quest trigger in Reunions, so if it doesn't work when you get there then you've missed a quest trigger and have a problem.
To get inside, you can either take the roof entrance or go in through the basement.
The roof entrance of Fort Hagen is guarded by a bunch of turrets. You can completely ignore and avoid these by using the door in the Parking Garage next door. Go down the stairs. You'll find a hideout room with a dead person and some supplies. Leave the room toward the main part of the garage, and you should find a blue door on the same floor.
Go in, and it should complete the "Follow Dogmeat" objective.
Load and Try Again
If you find that none of this works, then you'll likely need to load a save from before the robot.
If you're playing a Bethesda game and you don't have a trail of saves to go back to, then you're doing it wrong. Anytime you do something you don't want to do again, save.