bent valves on all cylinders

Hello together!

I've been reading a lot in the forum and have just upgraded my account because I want to ask you for ideas why an engine that has died.

I am a licensed engineer, but not that experienced on Rotax yet, even though I'm of the same country of origin .

Recently I was called to a "hard to turn over" 912S3 100hp. I recognized that besides very high friction during turning, there is a certain blockage to overcome. There was no oil indicated on the dipstick. After opening the oil tank lid I estimated minimum 1.5 liters oil missing. The pressure applied to the cylinders blew straight out of the carburetors and exhaust. So I removed the mugs and found stuck open inlet valves on all four cylinders.

The aircraft owner (a flight club) told that a member started the engine normally, but after e few seconds it stopped and the starter would not crank it again. The last flight was a few days before it without any occurrences. Multiple people are using the plane. Every single one declared to have checked oil level during preflight. The last 50 hrs inspection was 30 hrs ago, done by myself.

What I am really wondering is: How can that much oil be missing? There is no drop of oil outside on the engine, compartment and belly. If the oil was low before, why were there no signs earlier? I don't want to think about, but could it be that it was sabotaged and the oil was sucked out of the tank and only the last unlucky guy did not check? Or is there another explanation for the seized/bent valves and the oil was just low because it was not possible to turn the engine over until gurgling? They said the engine was running less than 10 seconds before it got stuck. Any advises were to have a closer look when I tear down the engine later?

Of course, I am waiting for a new engine now. There is no reason to waste more time on this one. But I am curious what happened.

Thanks in advance for your inputs!

Cheers, Uli