Speaking as both a cop and a geocacher, have patience with some of our (police) more uninformed bretheren. The job can really warp your perspective on things and most officers aren't real well rounded (which explains the high suicide and divorce rates to some extent). Calmly explain the purpose of what your are doing and offer to explain it further (and maybe even take them along) if they would like. Most will thank you and leave.
On the other hand, it is kind of fun when I get approached by security guards and the like who are wanabies, wanting to get nosy and I just badge them away.