THANKS FOR ALL THE REPLIES!
The errant ball solenoid is working now, although I can't say I did anything in particular to fix it.
To recap- I pulled the board. Gave it a stern looking over. Put it back it back in. Checked the resistance and voltage seen at the solenoid when the game was powered on but idle (attract mode, not in errant ball search mode).
While doing such, noticed that when the ball drained during attract mode the solenoid was firing when the ball triggered the ball gate switch upon settling in the normal ball starting/return hole. I don't recall if the game always did this or if it's a new behavior. Maybe I didn't notice it because of the sound of the ball rattling down the chute at the same time.
Either way, I now see the solenoid firing, albeit at a time I don't recall it doing so before. I take pcjohn's advice and jump Q2's metal heat sink tab to a ground post and away the solenoid fired, again confirming things were working. (It took me a little research to learn that on the TIP 120 transistor is electrically connected to its collector pin. When pcjohn said how to test fire the solenoid, I was a little confused at first how the metal tab was relevant. Thanks for the tip, John!)
So at this point, I know the solenoid isn't bad and nothing else seems broken so I coin up a game and drop the ball into the errant ball hole. BINGO, it works. The solenoid fires away and gets the ball back into the starting hole after a couple tries as it had been doing up until a couple days ago. I don't know if unplugging the machine from the wall, and/or removing and reseating the board "reset" anything but I won't complain with "just works again", I guess.
I am still a little curious if anyone can confirm that the solenoid is supposed to fire upon the when the ball triggering the ball gate switch when it drains into the loser hole during attract mode. I confirmed that it doesn't fire when I do the same thing during actual gameplay. Again, maybe it always did and I'm just noticing it now and mistaking it as something new.
As always, thanks be to KLOV!
P.S. Came across this resource while researching. Beside all the forums and YouTube vides, it was informative and likely will be helpful in the future.
https://www.pinwiki.com/wiki/index.php?title=General