So it seems I might have made a mistake.
It was sunday evening and I was hanging out with around 10 people aged between 16 and 26 (rough estimate). At one point I grabbed the phone of one of the persons and unlocked it with the pattern I observed previously (not intentional though). Then I went ahead and created a whatsapp group with nearly all their contacts, gave it a random name (no swear word etc.) and sent a message along the lines
I love you all -a few emojis-
Then I gave back the phone and waited for the group to blow up. And it did. Then they handed me the phone to say that it was me and not the phone owner who sent the message. But I sent a picture of the phone owner with the line
Maybe it was me who sent the message
The phone owner was fully aware that I was having their phone all the time, though I had no explicit permission. They also never asked for their phone back even though they knew I unlocked it.
As their parents were in that group as well, they witnessed everything that happened in there and now they might want to press charges.
Yet I am wondering what actual charges they could press. Can I be charged for theft? Can I be charged for intruding the privacy of the phone owner? Or something entirely different?
Note that no physical harm was done and as far as I can tell, the only cause was embarrassment. Also the phone owner is 17 years old, so not an adult according to german law.