ok so most ppl in any stalker games (mostly, CS, SoC and CoP) will either tell you to put your gun away or proceed to kill you if you dont, this took me a while to figure out when i was a noob but you can unequip a gun the same way you equip it, like the pistols key is 2 to equip it, same goes to unequip it :)

(this goes for all the weapons you have apart from misc items and food/medkits)