dutchy101 The boiler have welded in Incolloy 800 heating elements, I have heard once, of bad welding on the elements causing leaking, but this was immediate and around 8 years or more ago, very quickly sorted. So it won’t be that. If your machine is young enough to have drain taps, I guess one of those could be leaking.
However it’s highly likely the leak is as you said careless filling, or possibly water leaking from the top down, via the inside of the insulating jacket. vacuum breaker, safety valve etc..
I think you will be fine to put it on it’s side without draining and then check underneath war water tracks on the bottom of the boiler, or any obvious signs from any drain taps or heating element welds. My suspicion is you won’t find any weld leaks and it’s one of the other reasons.
