Was just in a match where three mechs were standing in the Sigma node but it wasn't recognizing any of us. I was in the node first, then two others came in. No enemies were in the node. After Sigma I ran to Theta and I was able to capture it.
This happens from time to time where the game/server thinks there is an enemy capping that location... whether they have died there and/or have touched it recently before its demise.