If it was planes: then the FAA would be looking up whomever it is and slamming them in court and otherwise. Look at typical airspace violations – they pounce on the offenders extremely quickly and there are immediate consequences. There is an immense amount of radar coverage over the US and any “mystery plane” not flying with their ADS-B transponder can be met with a response up to and including fighter jets being scrambled.
Regarding drones, it’s pretty much a weekly occurrence that some numbnuts flies a drone in restricted airspace. The FAA have gotten very good at tracking down these folks.
So basically, both of your theories happen routinely and our government is very practiced at dealing with them.
Sure, maybe at low altitudes, but try that at 1,500 feet.