Question Summary:
Are the people who have overheard the proposal and acceptance of Nikaah considered witnesses?

Question Detail:

If there was a haraam couple having a conversation and male asked the female hand in marriage and she accepted and there was a declaration etc (that makes a nikah valid) however they didn't think anyone was hearing their conversation, yet there were people hidden over heard the whole thing (1 male 2 females), and the couple don't know about them witnesses and will never know then is their nikah done? 

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Marriage is a sacred institution. Through marriage, two individuals and families come together. It is an occasion of happiness and joy. The celebration of marriage is announced to the public, friends, relatives etc. Everyone gets together to express their happiness to the couples getting married. In such a situation, how is it possible for two individuals to propose to one another while no one was aware of the proposal taking place and the witnesses were hidden? Where did the proposal take place? How large was the area in which the proposal took place? If the individuals are genuinely interested in one another, what was preventing them from performing such a sacred act in public?
 
And Allah Ta’āla Knows Best
Mufti Abdul Hannan Nizami,

Chicago, USA
Checked and Approved by,
Mufti Ebrahim Desai.