Temp sensor is pictured in Hamster-GTs photo (first one above) - it is attached to the bottom of the metal bumper - just to the left of the picture's center, highlighted by the intercooler behind it.
It obvious the Forte uses something other than the horn when you lock it just from the sound. Why not just chime the horn quickly like every other car? Who knows? Automakers do questionable things like this all the time. You just hope it's nothing too mission-critical they decided to weird out on.
Yeah that was gonna be my next guess, since I found it looked a bit like a little speaker and due to its placement.
I tried to google it last night to get a picture, but I couldn't even imagine what the name of it could be, and googling "door lock sound" and "keyfob lock/unlock noise speaker thing" didn't yield any useful results so I gave up.