@Chris
Erm, I know I'm primarily an ArmA chap, but I do know that createVehicle and deleteVehicle work fine in OFP (though I think there is a more advanced version of createVehicle that doesn't). createUnit is for creating AI, like soldiers, so I don't think it is the right thing to create "dumb" targets. The examples (both of which I've tested with a "Jeep" in OFP Res 1.96) assume that you want the target to appear at a marker called "targetMkr" and you will need to replace "invisibleTargetClass" with the actual class name of the invisible targets that you are using.
; Create at start of mission
_target = "invisibleTargetClass" createVehicle (getMarkerPos "targetMkr")
~120
; Remove after 2 minutes.
deleteVehicle _target
~180
; Recreate at 5 minute mark.
_target = "invisibleTargetClass" createVehicle (getMarkerPos "targetMkr")
or, if you are going to be removing and replacing the objects a lot, it might be better to just move them away and back again,
; Create at start of mission
_target = "invisibleTargetClass" createVehicle (getMarkerPos "targetMkr")
~120
; Move off the map after 2 minutes.
_target setPos [0,0,0]
~180
; Move back at 5 minutes into the mission.
_target setPos (getMarkerPos "targetMkr")