This is what i like to hear

What do you understand by the term professional witness in this context?

someone who is paid to gather evidence and present it in court

kind of like a prosecutor, private investigator, security guard and a while host of other "private" witnesses...

no not like a prosecutor who is working for the CPO and is trained regulated and constrained in what he/she can do.
but yes like a private eye who can also be very dodgy.
security guards.....what can I say. I've had run ins with some of the more meatheaded jobsworth variety who are usually 'only following orders'.
I wouldn't like to see the role of private security guards extended any further. They often have a very tenuous grasp of their own and other peoples rights

guardian today............

"Tishman Speyer paid $5.4bn (£2.8bn) in October for more than 11,000 apartments clustered in 110 tower blocks along the east side of the island. There was speculation from tenants' groups that the firm would seek to end controlled rents in many apartments to get a return on its investment. The New York Sun yesterday revealed that the company has brought in private detective Fred Knapp to find tenants in breach of rental controls

Mr Knapp is well known for such work. For his fee of $1,000 to $2,500 per case tenants on low stabilised rents can be evicted. Once an apartment is renovated it can be let to new tenants at uncontrolled prices, usually twice the amount.
Tenants in Stuyvesant Town complain that an atmosphere of suspicion is being created. "The sweep is so wide that legal tenants who have done nothing wrong are bearing the annoyance and expense of defending themselves," said Daniel Garodnick, a member of the city council who lives in the complex."