Getting strange indicator
 

I have a 2015 CRV with 45k miles and keyless ignition. Yesterday when I was driving it there was an indicator that came on the said KEY batt. The batt was flashing. I googled this and one said it was the battery in the fob. So I tried my wife's fob ...same thing. Hooked up the battery charger and the indicator went out. Left it on charger for about 30 minutes. Today I had the battery tested and it was fine. Also had them check the alternator and it was fine too. I have since changed the batteries in both fobs.
This hasn't re-occurred today............but what is/was going on?