Before you get started on working with smell, I don't think signalling hordes is going to be a good match for smell.  For that I think a map would be the most appropriate.
Here's how it would work:
Every turn, every player (player or NPC) writes the current turn and maybe a scent strength to the overmap tile they're in, possibly with some exceptions (in a car prevents it, no writing scents to water type tiles, fields might store scent more strongly than roads, etc), it might also use the player's current scent attribute, which can be manipulated by a few things.
Every horde turn, each horde checks the scent of the 9 tiles around them and compares it to whatever else they're doing.  At this point the code can compare the timestamp of the scent in that map tile to the current time to decide how fresh the scent is.  Possibly we'd have a separate scent strength value that would make it easier to pick up, but wouldn't prevent them from following the trail in the direction of newer scents.
If they're hot on the trail of a noise or recently saw something, they'd ignore the scent, but if they're just wandering around they'd be eager to follow it.  If they decide they want to follow the trail they'll move in the direction of the freshest scent.