Given the issues with 912's and oil changes, air in the system, purging, etc. - I would recommend you purchase an oil system purging/priming kit
They are available from most of the Rotax parts suppliers - CPS, Lockwood, etc. I've used one for years on our 912 equipped Diamond and it helps ensure that the oil system is properly purged and primed after an oil change. You could use this to pressurize the oil tank before you drain it and check for a leak at the drain plug boss. In my opinion, you will want this kit anyway for future oil changes - go ahead and get it now for troubleshooting your current leak.