Whether they marked anything doesn't have anything to do with an easement nor does the location of the origin and destination mean that something is/is not an easement.

A utility easement can be implied - that is, not written and recorded at the land registry - because the presence of something (like existing utility lines) implies that an easement must have been intended whether or not it was recorded. For utilities like Comcast, these are permanent/perpetual easements because utilities are public services.

If they don't have existing lines, then, yes, you can force them to seek an easement.

If they do have existing lines and the easement was just never recorded, you can negotiate with them still. For example, you can ask for some standard of how they return the property to you after add their lines. At my family house, we had asked them to use matching sod instead of just patching things over which they happily did.