Mysterious Footsteps

I was all alone on a rainy night, and my mom and dad were at a movie and my sister was babysitting. I was holed up in my room with the door shut just messing around.
I went out to the kitchen to get a drink, and before I went back to my bedroom, I let my cat inside the house and left on the kitchen, dining room, and hallway light.
About twenty minutes later, I got hungry, so I went back to the kitchen to get some dinner. When I went in, the lights were all off and my cat was back outside. I just thought someone had come home and I had not heard them come in, but I looked and no one else was home. I was 100% sure I had left the lights on, and I know my cat sometimes will tear the screen door and get outside, but she could not have because the glass door was closed the whole time and there was no tear. Besides, she would not have wanted to go outside because it was raining, and like every other cat, she hates being outside when it is raining.

I was a little bit nervous, but I just ignored it and let my cat inside and kept eating.
A little bit later, I was very sure I heard footsteps in the hallway, so I looked, but no one was there. About this time, my dog, who had been sleeping the whole time, woke up as I got back in the kitchen. She was acting like there was someone else in the back of the house, so I went into the hallway again and she followed me. I looked in all the rooms, and she was totally perked up and acting like someone was in my mom and dad's room. As much as I looked, no one was there. I was still nervous, but I just went back to the kitchen to finish eating.
Five minutes later, my sister got home and I told her what had happened. She stopped to pet the cat and dog, (the cat was inside) and we went to check my mom and dad's room one more time. Just like every other time, no one was there. We went back to the kitchen, and what do you know, my cat is back outside again. We never figured out what happened, but I know for a fact nothing was just my imagination.