Who was playing hide and go seek with my son?

Yesterday was rather uneventful.
I went in for a standard tire purchase.
Ended up having to replace shocks and struts as well.
Not complaining because my car drives so smooth now...but anyway

Hibry and I arrived to mom's house a little after 6.
My niece Savannah and her friend from school, Avery, were there to greet my son.
They played outside, ran around the yard, then were seen playing in the living room.

They stopped running around to grab a bite to eat before playing together once again.
Meanwhile, I was piling food into my gullet because I had managed to forget to eat all day over by the computer while I filed my mail-in-rebate for my tire purchase ($140 gift card just for buying 4 tires, btw)

I saw who I thought was Savannah dart behind me into the bathroom.
then, my son and Avery came running from across the other room into the bathroom behind me.
They called out Savannah's name, but got no response.
They retreated to the living room to watch some tv.

Then, that same figure darted back towards mom's room.
Hibry and Avery chased after her.
Again, they called out my niece's name, Savannah.
Got no response.
They turned on every light.
They looked in every closet, under all the beds, looked in the showers of the bathrooms.
Couldn't find her.
Mom said "Wow, she hide real good this time."
So we all started looking for her.
Mom, me, Avery, Hibry.

We gave up and were sitting in the kitchen.
Then my stepdad entered the house and asked why we were yelling for Savannah since she had been outside jumping on the trampoline the entire time waiting for them to go outside and join her.
Our jaws dropped.
Chills shivered up our spines.
the kids put their shoes on and darted outside to join Savannah.

Mom simply got up from the couch and demanded my stepdad get more Crosses and place them in every room.

So who was playing hide and go seek with Avery and Hibry if my niece had been outside THE ENTIRE TIME??