Referencing the Wikipedia that you tagged, "To have reasonable suspicion that would justify a stop, police must be able to point to “specific and articulable facts” that would indicate to a reasonable person that a crime has been, is being, or is about to be committed." A reasonable person would argue that recording the emergency backup generator, the gas pump and the prisoner loading sections of the Police Department does constitute specific and articulable facts that would indicate that a crime is about to be committed. It isn't a huge logical jump to assume that the purposes of recording the proximity of the gas pump, backup generator and loading dock may be to plan for a car bombing to take out the gas pumps, electrical system and secure the release of prisoners at the same time. It could be that he is doing recon work for a prisoner escape or a potential terrorist act. Who knows. No matter what, it's sketchy and without a halfways decent reason, it shouldn't be a surprise when the officer takes it as a serious threat.