I took off, and made a beeline straight towards some antennae (which I am learning may not have been a good idea?) that were about 2.5 miles from and 1300 feet above the home point. The drone lost connection at about 8300 feet from me, so still more than a mile from the antennae.
At a mile away, the antennas had no effect on your Phantom.
They are irrelevant to the incident.
It does not seem to have attempted a return to home.
It may have but because connection was lost there is no record of it.
Curiously, when looking at that area in the controller now, the setting isn't even there.
That's because that setting (like many others) is stored in the Phantom, rather than the controller.
It has to be so the Phantom can RTH if the connection is lost.
The reason I pushed the automated RTH,
If you pushed the RTH button after losing signal, it would have done nothing.
If you pushed the RTH button shortly after signal has reconnected and the Phantom is in RTH, you would have cancelled the RTH.
So perhaps I did not make sure the battery was securely in place? While this is certainly possible, I have always tried to make sure it was clicked in place securely...
The battery was probably secure.
The batteries are well designed to lock in properly.
Although this possibility is commonly brought up in forums, actual cases of them falling out are very rare
the main question I have at this point would be where a good point to find the craft would be. Flying at 40mph, with perhaps a 5-8mph tailwind, and considering it is 524 feet above the ground
No-one has mentioned wind so far. You suggest the possibility of a slight tailwind.
Was there a 5-8 mph tailwind where you were flying from? How accurate is that estimate?
If it was 5-8 mph on the ground, it will have been significantly higher 500 feet up.
If it was a tailwind going out, it will have been a headwind coming home.
The Phantom drops back to a lazy 22 mph in RTH.
22 mph into a significant headwind is not a good scenario when the Phantom is 8800 feet away and high up.