Native English speakers: grammar question

[Tarbicus]

Nathan's got it, as well as Ian, but I might put a semi-colon instead of a comma.

"No, they left the house this morning; I don't think they are back yet."

Or, split the sentence into two.

"No, they left the house this morning. I don't think they are back yet."

"...in the morning..." implies a future event, specifically tomorrow, whereas "...this morning..." means earlier that same day.

A general rule I was taught at school about punctuation - if you speak it out loud and need to take a breath or pause, that's where you'll find a comma, or some such.