Well, I could try testing the FC/CC levels again tomorrow morning, just before the pump turns on, to see if the FC level is lower and CC level is higher than it usually is. If that's the case, I'd rule out the theory that it was a one-time, localized contaminant and I'd favor some other pool chemistry is at work when the water is still. But if the FC/CC levels are the same (or nearly so) as when I take a reading with the water moving through the pump and filter, then the theory that it just happened to be some localized occurrence would seem to be more likely.