Use a conformal coating, this is used for Marine electronics and "permanent" connectors. The PIR "lens" is probably OK outside, unless the device is in the baking sun all the time, however the external circuit and connections can collect humidity and therefore corrode, so the conformal coating is pretty effective against that.
I have found that silicone can affect sensitive circuits (due to capacitance?).
http://en.wikipedia.org/wiki/Conformal_coating
Don't breath in the magic smoke!