Sometimes I find myself running around the sectors, but can't seem to find the last one! Help please!

You could build an X-ray Detector

X-ray Device = FumblePak + Pack of Gum + X-ray Tube
Display Unit = Lameboy Display + Copper Wire
X-ray Detector = X-ray Device + Display Unit

X-ray Detector + batteries kan be used to find nearby enemy on the map
Save before merging items, and use a merc with high mechanical stat and electronics skill

I use quick save and then let my mercs run around to find him.

If you're using the 1.13 mod, you can check the mini map (press [Insert] ) if there's only 1 enemy left, part of the map will be coloured red. this gives you a rough location.