If your hero finds a long long 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.
Please note: Being correct does not invalidate this trope. Sometimes ancient artifacts ARE weapons, after all.
- Mass Effect has an alien beacon which "probably" contains instructions for building a weapon.
- 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.
- GTO's Onizuka goes on a treasure hunt and finds what is obviously an ancient treasure worth millions. Unfortunately, this one WAS a weapon (a bomb).