Where( )about(s) is a local variant on where as used in a question.
It means 'give me some idea of the general vicinity'
- Whereabout were you when you realized you didn't have your phone?
Some would say whereabouts. There's no difference, but both are stressed on the last syllable.
The other whereabouts (always with -s) is a noun. It refers to someone's present location, and the person is indicated by a possessive:
- Bill's whereabouts is/are unknown. (some say it's plural, others don't)
This whereabouts is stressed on the first syllable, not the last.
Executive summary: WhereabOUT were you? versus His WHEREabouts.