I think Jay J hit it on the head. Although I do know there is padding underneath; I was here during the installation and I can tell as I walk on it. (It's a big difference from the old padding which had been under there for decades.) It appears to be a line of tape and it lifts the carpet at the seam just enough so that light coming in at an angle makes it more noticible.
As for going to the salesman, well, like I said this is a small town. The salesman and the installer are one and the same. I'm not too worried about it; I'm more worried that the boss, who doesn't live in the same city, will want me to follow-up.
We were quoted and chose from the samples a "20 oz commercial level loop." According to the bill, the carpet was "Mohawk Interaction #1631/Northport, color #144 Canadian Rocky/Tempting Teal," and there is a synthetic pad.
A special thanks to Jay J for his E-reminders to check back. You don't have to bother anymore, Jay. I'm going to remember this place is here. Actually, I've already bookmarked it. I'm very impressed by the little "BV" icon.