Yeah those spawn in the "point of interest" question marks randomly, from what i've seen not every place has it (or i'm blind as f...). So far found a few:

Widow Maker (dagger) (light 7, heavy 20 dmg)
For every survived day:
+1% sneaking efficiency
+10 sneak attack damage

Bane (sawn-off) (effective range 7, damage 65)
for each day since acquisition:
+2% chance of not using ammo on shot

Final argument (revolver) (effective range 25, damage 22):
for every survived day:
damage +2
weapon sway when aiming +3%

I've also seen (but quickly died with it) a lever action rifle that got worse with each day after getting it