Lamp posts usually belong to the utility/municiipality and are located within their easement. Looks like this would be a private contract between the owner of the easement and the cell/wireless provider.

Whether it would be a violation of the terms of the original easement would be a matter for the orginal grantor or their successors to determine. For example, if the easement was for water and sewer and now a 'lamp post' was being installed.

Should not be a need for a search unless there is the need for a new easement or a question as to the original easement granted.