The closest thing for me was a neutral race that picketed my planets for a while. I had no way to fight back, so I just hoped they were being curious while I tried to build some ships. Everything went well, until they got curious about how kinetic bombardment weapons would affect my population's health. That was a very fast game.
Building PDCs sounds like a good idea to me, if just as insurance in case they become curious in the same way as my NPRs did.