I am not a programmer, and I searched the forums to see if I could find a plugin for sentries, but could not find one.
This is what I am currently using to show sentries placed by other players. I made it by copying the default sentry plugin and altering it to work with sentries from other players.
I know it needs a few things added such as first disabling the default sentries plugin, making it only applicable in GRs, Color changes, or other things perhaps, but I am not bothered to mess with it any more since I have no idea what I am doing.
Feel free to correct and improve it if you know how, and repost, but I thought I would share what I have in case it is useful to others. It would get placed in the "Plugins\User" folder.
GuardianSentry.cs - Pastebin.com