While on Marine I was shooting my default starting weapon when I noticed cracks appearing in the wall for a secret room. The cracks appeared only after I had shot the wall with the starter weapon. I think the explosion created the cracks, and only the explosion, but this may be a great way to find secret rooms without wasting other weapon ammo. I will keep testing to make sure it is only the explosion causing the cracks and not on every shot fired from starter weapon after picking up this passive.
Update: I shot at walls with the starter pistol over and over till I found secret rooms. I've determined that with the passive item only the explosions from the starter pistols shots can create cracks to signify secret walls. I'm not sure if secret walls are considered a spoiler, so if you do know for certain please update the page with this information behind a spoiler section. I'm not versed in making these changes, I just wanted to add this contribution and test it.