What if they were generating private keys from a single seed stored on their server, not necessarily keeping them one by one on purpose? Maybe they actually did delete the sessions but weren't smart enough to not generate them from a single (kept) key? The police could then just generate as many keys as they can with that one seed on the server...?

Last time when the police seized a DNM, they actually spent some time as a MITM collecting information and they announced that publicly. Maybe if CM kept logs they would also tell the public about it?