That certainly sounds like the case except the water surface area is 5" x 4", which is consistent with OP's statement that it is "very low".
The only thing left in my mind is something that may not be likely, if even possible with this particular toilet, and that's an internal crack large enough not to let the level rise and then fall very slowly.
I called and was on the phone for close to 10 minutes with American Standard. They kept telling me that they have steps they would talk you through to try to raise the level. When I told them that we added water directly into the bowl and it didn't rise and slowly go back to the 5"x4", the person insisted on going back to there normal trouble shooting method.
The person did say that they have seen internal leaks that resulted with water on the floor as well not showing up and going down the drain.
Hopefully the water was added very slowly, like a cup at a time, or so.
EDIT: The only reason I called them was to find out if they had a cross section of the trap way I could get to see. They said no. I don't believe they don't have it but I guess not that I can see.