OMSA should not be trusted with the S-series chipset controllers. I suggest that you verify what the client told you by booting into the controller BIOS to check the status.
If a drive falls out of an array then you should run diagnostics on it. If the drive passes diagnostics then rebuild it back into the array. If it fails diagnostics then it should be replaced. You should always clear hardware logs before running diagnostics. If there are errors in the hardware log then diagnostics will usually not run correctly.