What does this barking mean? (Babysitting a dog)

I’m baby sitting a ten year old small dog, I don’t even know what type of dog it is.

She’s been well behaved for the most, but a couple times a day she will just randomly stare and bark at me. If I try to pet her she will just back up. She’s been let out recently, has food and water.

My guess is she just wants to play or wondering who am I and where is her normal owners?