The resolution
So here's the rest of what happened...
Sorry to keep you all waiting so long, I didn't feel well when I got home (think I'm fighting a cold) and I just went to bed.
It's been interesting to read the responses!
I quickly realized that I was in one of those awkward situations that we can so easily find ourselves in. I knew there was a strong possibility that someone was hurt out there in the dark BUT I also knew that I was out there alone with no quick backup and I simply did not know enough about the situation to go investigate. Also, I had just locked the shed and the back door to the house was locked so there was no quick way to re-access the phone or turn on the yard lights without both turning my attention away from the intruder and giving away my presence.
So I cleared my cover away from my sidearm (I was ready, but did not draw it) and then "spotlighted" the guy with my Magcharger (a powerful light, which is somewhat upgraded thanks to the wonderfully loony folks over at the Flashlight Forums) and in that strong "command" voice said...
"Mister, don't jump over that fence or my dog is gonna be REALLY upset".
He froze and just stared into the light for a few moments like he couldn't figure out what was going on, then he cussed at me and did an about face and headed off across the golf course. I did not pursue, but stayed put and I really thought he was going to get away but about then the cops arrived and detained him.
With me indicating where I'd heard the scuffle they also found a young woman laying dazed (and mostly stoned as it turned out) on the ground.
The whole thing had been an attempt to work out a drugs-for-hooking deal but both parties were stoned half out of their skulls and when they couldn't come to an agreement the guy slugged the girl. Both of them had drugs and paraphernalia on them so they all got some free bracelets and a place to stay for the next few days courtesy of local law enforcement.
The guy wasn't "officially" armed but did have needles and razor blades on him and, according to LE, had quite a history of getting violent over drugs so letting him close could have been a very bad idea.
when I expressed some concern about my property and person the cop chuckled a bit and said "he couldn't even tell us which park he's in right now so I doubt he'll even remember this in the morning".
Reviewing the whole situation in my mind I can only think of a few things I might have done differently.
1) Worry less about interrupting someone's innocent tryst and hit the yard lights sooner.
2) Have my cell phone in my pocket, or keep the cordless phone on my person when I investigate.
3) Go inside FIRST and alert my wife that I'm checking something out so she can "back me up" with the phone and observation.
Had he jumped the fence things would have become EXTREMELY complicated because I knew that somewhere in the dark behind him was another person (injured or not, I didn't know at the time) so rule #4 was right in the way.
However the mental plan was to draw and go into a "harries technique" hold with my light, advise him that I was armed, and order him to leave. If he did so I would have let him go, if he continued to advance then things might have gotten ugly fast.
As you can all imagine, I am OVERJOYED that he decided to turn around and walk away!