Please help...Lost Puppy

I know most of you are dog lovers and will help. Our neighbour has lost her Chihuahua puppy and is desperate to find him. 

Yesterday, she was sitting on the couch watching TV and realised he was gone.
She called out to him and he never 
responded. She then noticed the back door was open. 

She has been putting up signs everywhere in an effort to have him returned.....