The key term to remember for hidden cameras is "the expectation of privacy." You cannot have hidden cameras placed in areas where people have an expectation of privacy. So, where would a person have expectation of privacy? In your own home, in a hotel room, in your bedroom are places obviously you would have an expectation of privacy. Where do you not have such expectation? In lobbies, halls, "public areas", anywhere outside are places you have no expectation of privacy.
So, can you put a hidden camera in a bedroom to catch a cheating spouse? The other spouse can - but an investigator is likely to be violating the expectation of privacy of the cheating spouse. The other spouse can place the camera there, because there is no expectation of privacy from the other spouse in the bedroom. There is nothing preventing you from selling or renting a hidden camera to the other spouse to catch the cheater.
What about in a privacy fenced back yard? A person has some expectation of privacy in such a yard. Maybe not like that in a bedroom, but some. So if you have to use a UAV (unmanned aerial vehicle), climb a tree or use a hidden camera in the back yard - you are likely violating the "expectation of privacy." I did not mention UAV in pure jest, as making a UAV out a radio controlled small plane and a tiny camera/dvr is not difficult. However, if you can take video/pictures from some place another person is likely to be, you can take video and photos, etc. This may be from a hill above the property, a neighbor's yard, the street or any other type of place.
The point is, be careful when placing hidden cameras or conducting surveillance and do not cross the "expectation of privacy." If you are not sure - you are best off erring on the side of caution - especially if you are a licensed investigator.
Whatever you do - stay safe!