Thanks for the replies everyone.
yorkiebebe- i looked all around, but nothing..and she was staring at the wall that's directly visible from where I was sitting--and from where she had been laying before being 'startled' awake. yeah, definitely creeped me out! lol
yorkielover4lif- i believe this too. Just didn't want to suggest it as there may be skeptics here, lol. Hubby is a skeptic himself; he was snoring rather loudly in a deep sleep but awoke when Bella went beserk. I begged him to go out in the kitchen with me to turn off the kitchen light, lol. He grumpily obliged, but chided me for thinking Bella was barking at a ghost or spirit.
bloominsuz- LOL. It can be annoying, I agree--I feel terrible for my neighbors--but when I can't see what it is that bella is barking, sniffing heavily at, or following in the air with her eyes and her head, it scares the living bejeezus outta me.
snowcrustracer- We live in an apartment complex and they have Pest Control..however, that could be a possibility at my parents' house.
jeanm1963- I completely agree. It is very scary, because when bella was lying on my lap and sniffing the air, she would crane her neck to the side, the way she does when she sees or spots something that holds her intense interest..sorta the way she does when she sees a stranger come too close to the car, or another dog (or cat) while on a walk. And then she followed-with her head and eyes-whatever it was she was looking at, sniffing so hard and nearly fell out of my lap trying to lean forward to sniff at the air. I had goosebumps the entire time, and just felt based off of her tension that something was just not right.
Shorkie Daddy- LOL. Your little one sounds adorable. Bella is also mixed with Shih Tzu.

You're right that dogs can hear all kinds of sounds from far away that we can't. She will growl and bark in her sleep as well, but when she goes nuts barking I can sense the fear and warning in her bark that sends chills up my spine.
Reese1- Haha!! you are sooo right about that! Both Bella and Rocco do the same thing. Just yesterday Bella started barking at hubby's jacket that he always places over a chair in the dining area.
ritzydaisy- aint it the truth! Hubby and i jokingly threaten Bella that we're going to get a no-bark collar if she keeps barking at everything that moves outside the window
Sweet Puppy- I apologize for the goosebumps. LOL. Bella is very good at spooking me, I'll tell ya that!
Thanks everyone, and everyone whom i didn't get a chance to reply to. As soon as I got up and went to turn off the lights all around the house (with the aid of hubby; i didn't dare want to go around by myself after this latest barking incident), Bella calmed down and even approached the barked-at spot, sniffing around. Then she went right back to sleep. Who knows what she saw, or heard, but i'm hoping it will be awhile before she "sees" or "hears" it again.