Repairing Service Phone and Data Steal

Hello everyone and sorry for my question, I am not really familiar with all this stuff about hacking. I needed to give my phone to repair it's case which has lost it's adhesive and needed to be replaced, the guy said will take 10-15 minutes and so it was, but I am really worried about my data, I turned the phone off before leaving it there and even used an app to check all the activity and he didn't turn on the phone to try to break my password but is there some kind of other schemes like “plug” the storage into “some of their readers” and copy all the material – no screen lock or anything like that so they can steal my data even with my phone off and that app not detecting any activity from inside my phone if they stole my data via their plug method? I am really worried and I really don't have a clue, please if someone knows for sure about this to help, my sincere thanks