Reviving the thread with an update. Had to have the car towed home the other day after the battery light simply would not go off and it died. I put the battery on my battery minder for a couple of days and battery voltage was now 13.2vdc, higher than it's ever been. Normally it's only gone to 12.67 or so. Started the car and battery light on. Checked alternator output and it is 12.21vdc, also higher than it's been. To make sure my meter wasn't misleading me I checked the battery on my other car and it is actually low at 12.1. Guess that one needs to go on the minder! Anyway, thought I'd try something to bypass the ECM/IPDM control and disconnected the control signal to the alternator. No change. Reconnected the control signal. I went back through bank records (I unfortunately don't have the habit of tracking when problems start, something to change on my part, but I digress) and I bought a battery in April 2018 then due to this issue starting I bought an alternator, after which the problem went away for some time before coming back. I've probably mentioned this before so if I'm repeating myself, sorry, but I think it's time to try a new battery.
Connections have been checked. Fuses checked. Alternator always tests good. Battery also tests good, but I can't help but wonder if it can still have a problem that isn't detectable with normal testing tools.