Bizarrely, I've had a brainwave post-corona lockdown and cracked the case which seems obvious afterwards.

When this was in the room and no beeps were reported, it was plugged straight into the socket.
When we pulled it back to test it after water splashed on it, it was plugged in either on an extension or a UPS socket.
When the first Kyocera engineer saw it, it was plugged in on the same UPS socket and he also heard the beeps (he didn't link it to the power).
During lockdown, a second Kyocera engineer came. Due to restrictions, we moved the printer into a separate room for him to look at and it was plugged straight into the wall socket. No beeps.

Just pulled out a separate printer of the same model, and my coworker reported it as having the same error. Thought to myself I wonder if it's the power, plugged it straight into the wall and no beeps.

So it doesn't like being on a UPS most likely, which isn't surprising. No visual errors and the Kyocera engineer didn't recognise it either.