Walker spent Monday visiting with the Lions, who also entertained Dolphins free safety Brock Marion that day. Marion is also an unrestricted free agent.
The Dolphins were hoping to re-sign one of their two safeties, but are only willing to pay so much for someone at that position.
Arturo Freeman, who was slated to start last year before Marion unexpectedly rejoined the team, figures to step in at Walker's spot. The Dolphins also have had contact with the agent for former Jets starting safety Victor Green.
Other Dolphins free agents could soon follow Walker out of Miami.
Defensive end Kenny Mixon is said to be high on the Cincinnati Bengals' list of priorities, while fellow defensive end Lorenzo Bromell has drawn a lot of interest from the Minnesota Vikings.