There are no reporting requirements for a flyaway - that's only required for injury or damage as defined in 14 CFR 107.9.
Aside from that, if you suspect that an out-of-control UAV might endanger other aircraft then you could call the nearest ATC facility and advise them of its last known position and direction of travel.
It's up to the RPIC to know all of the regs. I'm not trying to be "right" here. I'm just stating the facts.
Operators must consider that the rest of the reporting requirements for serious incidents listed in section 830.5 apply regardless of UAS airworthiness certification. Listed serious incidents that apply to all UAS include the following events:
• Flight control system malfunction or failure: For an uncrewed aircraft, a true “fly-away” would qualify
. A lost link that behaves as expected does not qualify.
And let's say "professionalism" (i.e., knowing and following rules) doesn't really do it for you. Then consider that there is a nice drone out there somewhere with your registration number clearly visible on it.
Someone finds it and reports that they found it. The FAA gets word that you've lost your drone and they want to return it to you. When they call you, they ask what happened. You say, "Oh, it was just a fly away and nobody was harmed." Well, how do you know? You didn't even know where your drone was.
Now, imagine that you told them, "Well, I informed the NTSB and they decided no investigation was necessary."
I only know which scenario I would choose. To each their own.