Gonna go out on a limb and assume this is a bait-n-switch scenario. Where the guy is just showing what could happen if he willingly gives up important information like that.
Because having a guard, who knows exactly what that crystal does and sees through, try and steal in what I assume to be a guard house is rather poor writing. Not saying this is stellar to begin with, but that's dropping the bar pretty damn low.
To be fair.... while the MC is being dumb... he has just transferred into this world....
And is this the "I'll pretend to be bad to teach you a lesson" trope??? Or is he really just bad??? Cause both are so common it's practically a toss up at this point...
Even if we go by stereotypes, I'm not sure which one it is here.
Either the kind guard who is trying to test or teach the MC...
Or the evil guard who mugs unsuspecting travelers.
I'd go for the former. The second one seems a bit stupid in this case. That could work in a world where the guard could pretend the kid lying when he accuses him of stealing his stuff, but with magic lie detectors around, that would be dumb.
@Wyrm I wouldn't be surprised by the evil guard. I mean, its a "mythril" equipement free for him, and if its not "usual" maybe he think he can steal it and stop being a guard. Guess we will see.