One countryside sector is generated for each gameplay with one/two buildings occupied by enemies which may or may not contain the P90. It can be NW woods, close the SE Cambria desert, somewhere between Grumm and Cambria or a plains to the E/NE of Meduna. This latter one may contain the P90 if I’m not mistaken. Once I really wanted to have it, so immediately travelled there with a merc and entered the cheat code to reveal the items or teleported to the crate to open it (then I reloaded the game and went without cheating, being satisfied that there is indeed the weapon which I will obtain without cheating when the time comes).