After having several more KPs, I did the following:
First reseated the RAM - KP.
Took out both, put in Apple RAM - KP.
Used one Apple, one OWC - worked.
Put back both OWC but in reverse position (see below) - so far, no KP.
I noticed the following about the right slot (and I had noticed that the first time I installed the RAM): it has a different feel; the RAM does not get seated in exactly the same spot and the lever seems a bit looser. When I first installed the RAM, I had a problem - got the beeps and I had to reseat it twice before it worked. At the time, I noticed the difference in the slots, but did not think about reersing the RAM.
So I think there's the possibility that there is something slightly amiss with that slot and depending on how/which module you put there (or it could shift ever so slightly when I'm moving it around), it may work or not? This is a refurb unit.
Does that sound like a possibility?
I'd then call Applecare.
Your responses to the three beeps are ideal. I too had badly seated memory in one of my laptops, can't recall whether it was this MacBookPro5,2 or an earlier (first generation) 17" MacBook Pro.
Also I have attended to at least one machine where reordering the modules (without straying from spec) did inexpclicably seem to work around the issue. Maybe the reordering simply coincided with best possible seating for imperfect hardware.
I'm not familiar with AppleCare in recent years, completely unfamiliar with procedures for reburbished units, but it's certainly possible that — with or without refurbishment — there's a hardware issue.
I've had several refurbs from Apple - they are actually sometimes better/more reliable machines because there was something wrong with them initially and they got diagnosed/worked on/repaired/replaced in CA and are then sold in the refurb section of the online Apple Store (the only place where you can buy them) at a substantial savings. And Applecare is your original hardware warranty extended to a full 3 years of telephone and hardware/software support/repair/repacement, including in-home service for desktop units if you live within 50 mi of an AASP. My previous 24" iMac needed a logic board (or rather a new USB port but since they're integrated, I got a new logic board) - replacement done in my home compliments of Applecare (approx cost a minimum of $800).
When I have time, I will a) call Applecare and see what that results in; if they want me to take it in, I will put the Apple RAM back in plus the original hard drive.
I made a decision today to call OWC and we decided that it was best that a) they replace the RAM, and b) because of the problem reports re. Seagate hybrid drives, I should return that as well. So, I've reinstalled the Apple RAM and OEM hard drive and will monitor the Macbook's behavior. When the replacement RAM arrive, I'll install them and keep monitoring. I thought this approach was the best idea for the moment - waiting to call Apple if the KP's or whatever other problems persist.
Thanks for the help - much appreciated!