It's not official from the team quite yet, but I'm reasonably confident this means what you think it means:
Chase Thomas (@CTcard44) September 17, 2013
That's former Saints and Raiders linebacker Chase Thomas. The Stanford product was considered one of the better outside linebackers of the draft class, but he plummeted all the way out of it before the Saints picked him up as a UDFA. He was then cut. The Raiders picked him up. Then he was cut.
Thomas just isn't as impressive on tape as his college stats would indicate, but he's got ability nonetheless. He also played both defensive end and linebacker in college, so the Falcons can try to mine that ability and coach him up on the practice squad.
If Thomas is on the practice squad, that means a spot has been cleared by a call-up. I'm still guessing Patrick DiMarco, though that's not official yet either.