This Trope is Obviously a Weapon

If your hero finds a long-lost ultimate artifact, there's a chance that there's somebody else around that will claim that that thing is obviously a weapon. This guy will usually be militarily-minded and often, not very bright. Despite the name, the unknown thing could also obviously be anything else. Greedy people will assume "treasure worth millions", while somebody who's tech savvy might assume "ultimate decryption code" or something similar.

Whichever form it takes, there are two main components:

The person making the claim had almost zero actual information about the object in question; and
The person claims to "know" what said thing must be.

Of course, sometimes ancient artifacts do, indeed, turn out to be weapons.

Compare When All You Have Is a Hammer.

Examples of This Trope is Obviously a Weapon include:

Anime and Manga

In Outlaw Star, Aisha Clanclan assumes that the Galactic Leyline has to be made out of Dragonite, or otherwise affiliated with the substance.
Jinnai from El-Hazard: The Magnificent World assumes that pretty much anything mysterious is a weapon. Eye of God? Weapon. Ancient demon? Weapon. That fancy stick over there? Ultimate weapon. The problem is that most of the time, he's right.
Great Teacher Onizuka goes on a treasure hunt and finds what is obviously an ancient treasure worth millions. Unfortunately, this one WAS a weapon (a bomb).

Literature

In the Foundation cycle a merchant got in trouble with customs thinking he is smuggling a holdful of weaponry. Their suspicions were triggered by a power tool of "dice rails like cheese" sort - after one of them managed to turn it on while pointing at himself - so they weren't too far off.
In Merlin's Book of Amber, the Ghostwheel - was instantly recognized as a potential superweapon by everyone... except its creator.

Live-Action TV

Stargate was considered to be an ancient weapon at some point. Stargate: Continuum adds some Fridge Brilliance to it: It made hole in ship's hull.
An episode of Star Trek: The Next Generation has the crew racing some Klingons and Cardassians for what's believed (by the Klingons at least) to be an unstoppable superweapon hidden in the genetic material of various worlds. To the last two groups' disgust it turns out to be a message explaining that every sapient being in the universe comes from the same common ancestor. Especially Klingons and Cardassians.