Well, as luck would have it, I was able to talk to the manager in charge of maintaining the pond. Sure enough, about 4 years ago, he stocked 20 Carp, in addition to the Hybrid Sunnies, LMB, and Channels.

They never told me they stocked Carp!

So, now I'm thinking it was a Carp I saw, and probably have spooked a few. There's a lot of matted algae on the windblown side of the pond, a few times I've seen a massive disturbance when my fly line lands on the slop. I've always assumed it was a Bass, but maybe they were Carp?