Perhaps this is baked..
You can now buy a hollowed out coin that can hold an 8 GB Micro SD Card.
They need to be augmented with a small battery, RF transponder (for reading/writing data), inductive charger, and, of course, circuitry to do voice activated recording (and maybe GPS logging). Bonus
if the charging can be done through harvesting kinetic energy, like some watches.
These could then be released into the "wild" and record all sorts of interesting conversations, and be gathered later to be "harvested."
The problem is, now I'm paranoid that someone else might have implemented this idea already. All my change is going in a soundproof wallet!