I can confirm that Murphy's agent has been calling clubs to assess the "lay of the land" - whether the club has "forced Murphy's hand" by bringing up the captaincy issue I can't say, but his management has used that issue in their conversations with other clubs.
Assuming that being the case, Murphy would be somewhat naive to think that his captaincy was set in stone.
I'd bet it wouldn't be the captaincy alone and more likely SOS not being ready to discuss Murphys future post 2018.
Given the way SOS has been handling things with senior players, this shouldn't have come as a surprise to Murphy either.......if he is shopping around.