Extremely loud alarms that can be heard from a long distance.
They could be sounded by a supermarket, vehicle or store. (The power and distance of the sound would depend on the origin of the sound).
These alarms could howl animals, so if you see animals running away from the noise, you would know that the alarm was triggered.
They would also attract zombies, generating a large horde in the place, which would make it very dangerous.
Any more ideas?