Most power wheelchairs don't have a lock or key, so you won't be the only one without.
As others pointed out, you will be bringing it into lines to the point of boarding. It will either be left right where you left it, or a CM will move it to the unload area if that is different than where you boarded.
Since it will be inside the attraction, it won't be sitting out where people can mess with it. Guests will either be getting on or off where your chair is. I can tell you from experience that most people won't even see/notice the wheelchair.
One time at Haunted Mansion, I got off, but the CM forgot to stop the ride so my DH and DD could get off (she can't walk and needs to be lifted off and into her wheelchair). The CM told me to just wait at the exit for their ride car to come by again since I needed to help in the transfer.
I waited right there, in plain sight, or one whole ride cycle. Hundreds of people passed by me, none looked at me and I am quite sure none knew I was there.
They were absorbed in getting off and talking about the ride or where they were going next.
Several wheelchairs were parked in the area. The only group I saw who noticed any of them included a little boy, who was looking his parents and almost ran into a wheelchair.
Also, the average guest will not have a clue how to turn the power wheelchair on.
I also would STRONGLY suggest against trying to put a lock of any kind on it. Anything that prevents a wheel from moving could do serious damage to the motors if someone tries to move it.