Gym "Rings" (not a Q?, just confirmation)

First time poster & long time reader - so thanks to this community and this site for all that you offer; I've learned a ton and visit every day!

(tl;dr: go to end. This could be kind of a story...)

I'll call it an "observation" on gyms from the overworld map. Certain gyms I see have white rings around them, but not all (4 of the 9 gyms from my house). I wondered why. Perhaps this has been addressed elsewhere, but all my internet inquiries took me to never-spun pokestops. This is not the case.

I thought it may be for upcoming raids (aka: "taking this gym for a near-future raid would be advantageous = more boss balls"). Or some behind-the-scenes code for where Mewtwo may have shown up, but is on hold.

One gym I was looking at had a ring. Shortly later the ring was gone. I went to 2 friendly gyms w/o ring (I was in the first, not the second). Fed berries to the first and placed a quality Snorlax in the second. They both now have rings. Hmmm...

Tossed about speculations with a nearby friend. Wasn't badge-related. Not team-related. Wasn't to each individual: we saw same ring/not-ring. He speculated cumulative 3,000 CP. I pursued that theory...

Checked my gyms. Busted out the old calculator. 3k wasn't the number, but thought he was potentially on to something.

Return to the gym that had a ring, but then lost it (I'm in it). I remote fed berries till cumulative CP was over 4k, and it got a ring. Waited. Ring disappeared. Calc check: just under 4k. Fed the toons (painstakingly, at single-digit increments sometimes) over 4k...and Behold, the gym got a ring! Waited again, and the ring is gone at 3,962 cumulative CP.

(tl;dr & summary) My best guess: gyms with a ring have a cumulative 4,000 or greater CP. If you are looking for a quick take-down, go for those without the ring. Confirmation of my findings is awaiting from the community.

And if this is the first-ever unveiling of this, gimme some credit! -TB