Most functioning implants are not made of terrestrial materials. They cannot be seen on X-Rays, MRIs, etc. because they appear on these to be a normal part of the body. The human body at the direction of our own DNA sequencing does maintenance on them. We all have a certain amount of alien DNA and have had for millennia. The discovery of DNA assumed that if it was found in all humans it was human. That is not true. A good bit of our DNA is of Alien origin. Since they don't understand the entire DNA sequencing line, they call some of it 'junk DNA'. It is far from that it. It is that DNA that enables some people to communicate and see in a manner that others cannot. Over time this 'junk DNA' is incorporated into what our scientists believe is normal DNA. Some people have more of this DNA than others do. It is a matter of learning to use that DNA. Fear will stop the body from doing so...just as fear can cause diseases if left unchecked.
The Visitors are perfectly capable of enhancing these implants if they need to without invasion of our bodies. They are there for communication rather than tracking. They have other technology that gives them the ability to track any member of any species on this planet.
They are a far more advanced species than we are...yet we continue to try to put their abilities and motives into our own small Human boxes where they don't fit. This is for the most part due to our inability to see and understand outside the box that our scientists have put us in.
This is our shortcoming, not theirs.
Attempting to validate their reasons for being here based on books such as the Bible and other scriptures is to assign them the same narrow sight that we have come to depend on to answer the questions we are either afraid to ask or afraid to understand. If you want to be realistic about Jesus, his 'virgin' birth would more than likely make him a hybrid than a god. We were shown something and have, in our usual selfish human desire for power/control/wealth dismissed his original teachings, written a book and made money off of his teachings. If Jesus were to come to Earth today, he would either be killed or put in a mental institution for insanity.