[xjcsa]

They have RFID tags the size of a grain of rice. Plant them & send them on their way . . .

That'll work great, as long as you can get an RFID scanner within an inch or so of the tag. Works for identification, but not for tracking.

[BraveMan]

Granted, you wouldn't have real-time data. Typical ten meter read distances would make urban tracking possible though, especially in a hot zone . . .

A few rice meals before they leave camp, and a few tags are ‘bound’ to remain; others ‘dropped’ along the way would provide an interesting trail to follow.