There's a big problem with surveillance: as far as I know, there is no independent monitoring authority in place to make sure the information that is gathered by investigators isn't being used by members of these agencies for their own benefit. An agent might listen to a phone call by someone negotiating a contract or involved in a business transaction of some sort that would allow that agent or a member of their family to capitalize on that information. All sorts of situations can exist in just about any type of business from real estate to securities trading where an agent or a member of their family might gain knowledge that would give them an unfair advantage in just about any type of transaction or contract negotiation.

I think that after an investigation is closed and the person that came under surveillance is cleared, they should be informed that they were spied upon and told exactly what was heard and by who.

It's a very touchy subject. Remember, unauthorized access to personal information forced one President or the USA to resign.