I suggested that they may come down in meteor like objects.

A single 'base' might be a foot across--look like a meteorite--and have holes in it with sugar filled fluids to draw insects in. Instead of pollen, the nanobots attach--and direct the behavior of the bug--which bites you at camp--and returns with a blood/flesh sample under nano-control...

--And all you see is a rock with a wasps nest--with wasps that seem really smart...

Assuming artificial, autonomous nano-tech works on any usable scale outside the warmth of biological processes...that is.