This doesn't mean they won't sign with the team. It simply could mean they are exploring all their options before making a decision.
Porter played for the Broncos last season on a 1-year deal. He has been reported as receiving interest from his former team, the Saints for whom he was a Super Bowl hero. The Saints have also showed interest in former Raider, Nnamdi Asomugha. If the Saints were to sign Asomugha, it would seem likely they pass on Porter which would put him back in play for the Raiders.
Hixon was the first and only reported visit by a free agent wide receiver for the Raiders. The market is very thin and the Raiders don't have much money to spend so seeking other options is understandable. Hixon won a Super Bowl with the Giants in 2011 and could be looking for a team who would give him a legitimate shot at being a starter.