If that is the “mystery” aircraft, then it’s a Cessna 182 Skylane with a Flir Ball on the side. Flir is just an infrared camera used for picking up heat signatures of bad guys. Most likely some sort of Law Enforcement/Government Mission. Portland, Oregon Police Department fly one just like it.
Law enforcement pilot training, maybe? That makes sense. But why treated like ‘top secret’ then?
Interestingly, we have had a few evenings where a helicopter was flying in loops over the same several block area, different areas each time. The first time was dark enough that you could see they had a light beam. But the circular pattern was like a moving spirograph, not just a continuous loop. At first we thought it was surveillance of criminal activity but the flight pattern made no sense and it went on for way too long. Then it happened a few other times in different areas. My final guess was it was indeed probably surveillance in the form of acquiring imagery for later use or maybe testing something.