Adele had another reason to confiscate their phones. She had a hacker with her in the camp who cloned every single phone with her. Elijah was smart and there were chances he didn't come with the phone he worked with but no matter how careful a person was, there were bound to be mistakes.
You can tell a person from the content and contacts of his phone. People like Elijah would have little contact with the outside world, probably only with people he worked with to minimize exposure while normal people would have more specialization, introverted or not.
Hence, they would take their time going through all contacts so as not to miss a thing. This investigation is very important to not only Marcel but to the entire Luciano family as well. Elijah took out their own people and became a threat to them. They had to do something before the matter escalated.
