Although the German LE are referring to the phonecall on the 3rd May, I'm thinking maybe it's possible that photos/footage of MM may have been sent/uploaded from his phone that night? If they can prove he had the phone for the call, they prove he had possession of the phone not the gf. If any photos or video etc were taken, (possibly already found on USB) and they know he had the phone and was alone/with no witnesses for that night, they could link him through that. I believe we are only getting very small bits of information. JMO