I recently found a new mod that wasn't previously available to me- I can use it for most of the laser weapons I have and it didn't cost any materials to craft, I could freely equip it for a short time. The mod was called "Beam Focuser X+" and was considered the most valuable mod and gave the highest stats out of the available mods. I now can't modify any weapons with this mod, however the option still shows, but under required materials it says "none."

I'm assuming this had something to do with the quartermastery quest and finding new tech, since in dialogue in completing the quest you have the option to keep it, but I'm now curious if there are other unlockable mods within the game.

Has anyone else discovered this or something similar?

there are power armor paints that are unlockable, and an armor mod connected to a faction that I won't mention for fear of spoilers.

Look at the bottom of the crafting interface. If you highlight the mod, and you have it on you, the option should say "Attatch" instead of "Build". The None in the right side of the crafting interface simply means that the selected mod requires no materials to be made.

I'm aware of the armor mods. Generally, even if the mod says "attach" it would also say what materials it takes to build. Also, there's no option to build or equip the mod (Beam focuser X+) to new laser weapons (though I can unnattach and reattach the current mod), and no possibility to craft a new one since it still says under required materials "none." and it's greyed out so I can't select it. This mod was obviously unlocked through a mission I did, and was special in that it can't be crafted, I either have it or I don't. I'm just unsure (though I have suspicions) where it came from, and am curious if there are more unlockable mods that have been discovered for weapons specifically.

Check your own mod menu in the weapons workstation. for me, Beam focuser X+ is listed below Gyro compensating lens as the best mod available. I am very certain I didn't begin with this option as I've been using the weapons workstation heavily (one of my favorite aspects of the game)

Without knowing how you got the mod, I can't reproduce your issue, and I've never come across a "Beam Focuser X+", so I have no idea what's going on. It sounds like you're talking about a Laser Rifle, but for me, the second version of the Gyro Compensating Lens (not in-game, so I don't know what it's actual name is) is the last Laser Rifle Muzzle Attachment. I've never seen anything below that on the list, and you're not even sure where you got it, so I can't do the same thing to reproduce your issue. If you could provide more information, I might be able to retrace your steps and see what's going on, but I don't know if I'd be able to answer your question anytime soon; even if you do remember exactly where you got it, if it's in a faction line (for example), it may take time for me to get there.

Someone else may be able to provide you with the information you need, but you're going to have to provide more information than "I got this mod somewhere and can't put it on" in order for them to be able to help you.

well, perhaps I've been unclear. I don't think this is a bug or issue. I just haven't seen anyone post about this so I'm seeing if anyone else has discovered this or something similar, that is unlockable mods for guns.

If someone wants to try and reproduce it I'm 80% sure this is from the Quartermastery quest (brotherhood of steel). I think I was able to attach the mod AFTER I found the tech, but BEFORE I turned in the quest, so the tech was still on my character. I also used the weapon workstation on the Prydwen when I attached it, which may or may not be a factor.

I've only done that quest once, and only to advance the quest line. The item I was asked to get looked like a computer chip. I didn't realize that they could have you pick up mods.