lol sorry, we should have been clearer. No, do not put it in an init field. What you need is a little script.
If you're not sure how to do this there are lots of good tutes in the Ed Depot: start with snYpir's Friendly Intro to Code snippets and Johan Gustafsson's Scripting tute.
The summary is, create a text file with notepad. Call it marker.sqs (ensure it is not marker.sqs.txt). Put it in the mission folder next to the mission.sqm. Copy and past this into the file
; this script makes a marker appear near the postion of the hunted unit
; thanks to deaddog and macguba at OFPEC for help with this script
#loop
? not (alive hunted):exit
_px=(getpos hunted select 0) + (random 500) - (random 500)
_py=(getpos hunted select 1) + (random 500) - (random 500)
"huntedmarker" setmarkerpos [_px,_py]
~30
goto "loop"
exit
Now name the unit that is to be hunted .... call him hunted. In his init line write
[] exec "marker.sqs"
Now create a marker and name it huntedmarker.
That should be it. If it doesn't work try playing with it a bit. You will need to do plenty of playtesting to see if 500m is any good, you might need a bigger or smaller number.
If you get stuck by all means come back here and ask, but please spend some time with the tutes and command reference first - if you understand what all this means you'll be able to fix it yourself if it's wrong, and make it better if its working.