Today in a conquest game on the Terra Therma map, the Sigma node was not able to be captured from the opposing team. I was in Sarah's Jenner and a team mate in a Raven 3L was with me, neither of us could get Sigma to start changing to our team. The opposing team that had originally started at Sigma had a disconnected player. That player's mech wasn't anywhere to be seen though, but it might be relevant.
Here's a screen shot.