For those who are not aware, a plant is an asset who is there to gather intelligence. Tends to be that nobody knows who the plant is. Their partners may not even know. Was a rather strange definition I got when looking this up. "You’re a plant and from the seeds of your intelligence, a whole conspiracy might grow". So I think it's safe to assume that Cassidy was inserted into Trump's administration and got close with those at the top. That would to me appear to be the most obvious. I do have one more theory. But I'd rather see this play out before saying anything...