Magic Weapons

@Eltab good idea and I just thought what if you had this applied to say a Gauntlet/s that a PC picks up in a second hand armor shop for x-GP and finds out that the fancy filigree /etchings actually alerts them to said monster/creature and deals the +1 to punching... it could be applied to other visible to the wearer parts of their armor

AnimeSniper said:

@Eltab good idea and I just thought what if you had this applied to say a Gauntlet/s that a PC picks up in a second hand armor shop for x-GP and finds out that the fancy filigree /etchings actually alerts them to said monster/creature and deals the +1 to punching... it could be applied to other visible to the wearer parts of their armor

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A neat idea, but tactically dangerous if the _____ live in the dark and have Darkvision: they will see you - or at least your arms - from a distance.
But the surprise on the players' faces the first time it happens to them would be a hoot. (Evil DM grin)

A similar and not-dangerous version would be magic gauntlets that get "my fingers are asleep" tingly when insert daytime creature here (ex: tiger) are nearby.
Immature players? Underbreeches (part of a suit of armor) that smell like you are having very bad gas when a _____ is nearby. No sound, though.
Want to provoke a fight? The Armor of _____-Fighting puts out an odor/aroma like whatever a _____ eats when one gets near. Originally made to insure the success of hunts.

On the original topic, for a Pathfinder game I'd agree with one of the above posters and just slap an extra couple of hundred gp onto the cost of another weapon to allow it to glow in the presence of a particular monster - in fact, unless the one particular monster the sword detects is a major foe in the campaign, glowing in the presence of one single monster probably isn't even worth charging for and is just an interesting quirk (or maybe +100 gp or something).
In previous editions of D&D, a sword with some type of passive creature detection ability usually detected a creature type, such as humanoids or giants. Now, if the sword detects humanoids and can differentiate between them (i.e., it can tell you whether the humanoids in the next room are goblins or bugbears), that's a bit more useful and might actually be worth more money.

In any event, you should probably give the weapon a detection range of at least 200-300 feet or more in order to make it useful.