Other places are in between the refinery and its silo.

Another good place is in the power plant, just behind the control panel in the middle of the room. If they are in a hurry to find it, they might run right past it without looking behind them.

If you are playing a Nod, you could put it behind the nook in the wall of the baricks. It is on your left as you enter the bericks tords the back. If they are in a hurry to find it, they will just glance in the room to see if you put it on the End Game spot.