A WiFi battery powered dead drop inside a stone with a few months duration with a cpu and clock so it turns on for 5 or so minutes/day. Time can be changed. Dropper visits site - and has 5 minutes to drop data(can be increased). Spymaster has another changeable pick up window for his minion to get data.
All data strongly encrypted.
The low duty cycle allows long battery life as the off state waiting can be a very low draw.
Just a thought experiment, but I bet something like this goes on now?