5 batteries one red.

[Hoohaa]

I have a bank of 5 * 12v batteries. They are around 3 years old.
Bought and installed at the same time.
Sit at around 13.7v.
I have been very happy with them.

While replacing my sink I was able to get a good look at 3 of them and noticed that the middle of the 3 was showing red in the little window.
I checked all of the voltages and they where all the same.
Do I need to disconnect the battery to test it properly?
Is it likely that the indicator window is malfunctioning?
Can it damage the other batteries in the bank?
Is it common for one battery to malfunction when wired as part of a bank like this?
Thanks. Jeff