Question Summary:
Does a woman have to observe hijab if she is outdoors but there is nobody around to see her except her husband?

Question Detail:

Does a woman have to observe hijab if she is outdoors but there is nobody around to see her except her husband? Example - a private beach on holiday where there is nobody but them two?

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
The term hijab could refer to the outer covering of women, such as an abāyā, in which the form and shape of a woman is concealed. Hijab could also refer to the niqāb which is the face covering.
In principle, if the husband and wife are in privacy and there is no outsider (ghair mahram) around, the woman may remove her hijab.[1] However, a woman who is conscious of hijab will be, and should be, sensitive to the possibility of a man being around. This is especially true in a “private” beach which is not exclusively private.[2]
And Allah Ta’āla Knows Best
Hisham Dawood
Student Darul Iftaa
Chicago, USA 
Checked and Approved by,
Mufti Ebrahim Desai.