Why my dog Prissy appear by my computer room front door at 1a.m. stirring at me for about 15 second?

I know this is sound weird but I can't deny what I saw. Prissy died 4 years ago she was 10 years old and she knew I love her. When I was in my computer room, I thought it was Missy my dog now standing by my computer room front door stirring at me. When I looked down under my desk I see Missy is sleeping next to my foot. My friends told me that dog have spirit too.