Just triggered this on my S9 this morning by accident, apparently just tapping, not holding, power+volume at same time up turns it on and off manually on my phone. May keep it on for now that's a better theft deterrent than anything else on the phone LOL!
This didnt work for me on my Samsung galaxy S8+ but what unlocked it in the end was my alarm going off. The alarm seems to take over the phone and unlocks it when you x the alarm. So as a future fix Ive set my alarms to go off at certain times every day just in case it happens again.