I have made a mission like this a long time ago, I gave the player an "Examine" action which checked the ground for whatever objects, also its very basic scripting, here it is:
User action ["Rearm",User]
~1
?(User distance Key_Pos < 5):Goto"FoundKey"
titletext ["Nothing Here...","Plain Down",0.3]
exit
#foundkey
titletext ["You found a key! YEEEE-HAAAW!","Plain Down",0.3]
;then make whatever you want to happen here, such as make a vehicle unlock.
;also you will need to delete the Key_Pos as to prevent the player from finding the key over and over.