A spokesperson for Sound Transit was not able to immediately confirm, off the top of his head, when the Ts were applied to the building. "But it wasn't, like, two weeks ago, right?" I asked. "They're new?"
The spokesperson, Bruce Gray, confirmed it wasn't two weeks ago. It was more recently than that. It may have been today.
Asked what the T signifies, Gray said, "That is the regional T that designates that station as a regional transit hub." There are Ts outside downtown Seattle transit tunnel stops.