In the player's init field add
this setbehaviour "SAFE"
Even if he doesn't have a weapon, he starts the game out in an "AWARE" stance...I assumed you already had him starting out in "SAFE" so he looked like he was unarmed and just walking. My mistake.
If you want a more robust trigger, change the trigger condition to this and see how it works:
(behaviour player !="SAFE") && (count weapons player > 0)
The problem I found with this is - if the player is in SAFE, but then pulls a *pistol*, OFP doesn't regard that as a change behaviour. But if he is in SAFE and unslings his rifle, OFP does regard that as a behavior change. If the player pulls his pistol AND then kneels or goes prone, then OFP changes his behaviour too and the trigger will fire.
So it will work best with primary weapons only (no pistols).