I once saw something I can't explain. Had I been alone, I would've just told myself I was imagining things, but the fact that right after I saw it my friend goes: "Did you fucking see that?!" convinced me there really was something there.
We went back immediately and it was gone - despite this happening in the middle of an open field with nowhere for it to disappear to. Do I think it was a ghost? No, it was most likely a human. But it was an unexplainable, genuinely weird event. Having experienced something like that makes me a lot more sympathetic toward other people who claim to have seen similar things. This wasn't a floorboard creaking and my mind filling in the blanks. I absolutely saw a figure.
Use a pin code or a pattern. If you're concerned about privacy, then handing tech companies your fingerprint isn't very wise to begin with.