Cases of Mindless Behaviour

You ask your partner whether she bought the vegetables for tonight’s dinner. She doesn’t look up from her reading, and answers your question in a way you know makes it untrustworthy. She answers mindlessly.
Her mind is with her reading, but she can summon up just so much attention as is required to bring you on the running.
We easily recognise such mindlessness, but what does it consist in?

An answer to that question should help us understand just what happens when during a conversation someone you were talking to suddenly picks up his cell phone.