Some people do let their cats out at night and feed them in the morning when they come home. Cats make so much noise when in heat that some people let them out so thay can sleep!
Yes if you are feeling prominent nipples she might be pregnant and it could be she went off while in heat and got lost from her home. She may have a cozy spot to hole up during the day and just come by to beg a meal at dinner time.
I had a cat that lived with me, I owned his mother, and found out his time away was spent in a second home who thought he was their cat. They moved and took him with them! So cats are apt to seek out companionship, food, and 'better quarters' on their own too.
You might, if she would let you, stick a collar on her with a note asking her owners to call you. Or ask around your area to see if anyone owns a cat with her description.
Hope this helps you!
Cat Health, Behavior, Care Expert
30+ years cat owner, rescue, breeding, study of behavior & health care