Back when https://en.wikipedia.org/wiki/Anna_Chapman https://en.wikipedia.org/wiki/Anna_Chapman was in the news, I think I remember talk of hollowed out rocks c

Back when https://en.wikipedia.org/wiki/Anna_Chapman https://en.wikipedia.org/wiki/Anna_Chapman was in the news, I think I remember talk of hollowed out rocks containing a battery powered WiFi hotspot. Person sits on a particular bench at a particular time and a data exchange with their phone / some other gadget is triggered automatically. (Usual risks of the person setting the drop (and therefore the drop itself) being under surveillance apply, I guess)