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:
 * 1) The person making the claim had almost zero actual information about the object in question; and
 * 2) The person claims to "know" what said thing MUST be.

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

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 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.

Tabletop Games
"The Spelljammer? It's a weapon. Something that large, that fast, that powerful? Of course, it's a weapon. What else could it be? - Testimony of H'Carth, the Mad Beholder of Greyhawk"
 * The Legend of Spelljammer boxed set, "Legends and More" booklet:

Video Games

 * Mass Effect has an alien beacon which "probably" contains instructions for building a weapon.

Web Comics

 * Art and Pip in Sequential Art grabbed the most powerful ray guns of Denizens... or at least what they expected to be these.