"I would think that the second kind of activity could be detected by software."

Team play or multiple accounts being played by one or two people at the same table could be detected by software written to flag play by the same accounts at the same tables over time. I've seen collusion. Sometimes its obvious, sometimes it is more subtle and harder to recognize, but it does happen, too often in my opinion. I don't know what the priority level of prevention of such cheating on the poker sites is, but I don't think it is high enough.