Celestial Compass

Celestial Compasses are small lightly-enchanted devices that respond to the latent psychic energy imbued in the Risen Dead. They are used to get advanced warning of nearby undead, and are popular among the brave fools that explore the outskirts of citadels. Before the World Died, celestial compasses were used by pilgrims to seek out the divine gods, and could be tuned to track the location of a particular diety. They function similarly to magnetic compasses, but instead draw upon magical energy to continually align a tiny sliver of Radiant Steel against a gradient of latent psychic energy emanating from large pockets of undead. 
Celestial compasses are not full-proof. Firstly, most have to draw energy from tiny alchemical capacitors that only last hours or days at most, though larger compasses intended to run around the clock in one location can run off much more efficient energy sources- these larger examples are sometimes used as longer-range warning systems of approaching undead. Though still only in the experimental proof of concept phase, electromantic compasses that can carry a signal of undead strength have been developed, envisoned as a way to tell if railway water stations are safe to slow down for and stop at, but cost considerations and the need for a very long-lived power source have prevented this from being deployed out in the field. For a typical compass carried around in a pocket or backpack, however, it is not very difficult to switch out the capacitors with a spare if journeying somewhere potentially dangerous for a few days at a time. These are generally not too expensive, and see use as precautionary measures in places like the mountains around Acarletto- still generally safe, but you could come across an isolated count of undead even just a mile or two from the citadel walls.
The second problem with compasses is that they do not equally detect all forms of undead. They are most directly applicable to detecting the risen dead, who were long ago directly imbued with the psychic energy of the divine gods, which celestial compasses were originally invented to track. Others like emotion-wraiths or seaghosts are detected only at a notably shorter ranger, while others like vampires are not detected at all. Unhealed are also in many cases picked up by the compasses, which may invalidate any strong signal put out by the compass if one is nearby. With all this in mind, celestial compasses are still considered essential equipment for those outside of a citadel, but cannot be the only form of vigilance one partakes in.