As someone who's worked at two Zs, I've noticed Tracons *LOVE* flashing airplanes coming to me practically as soon as they get them. In fact last summer I had a departure aircraft flashed at me that didn't even have altitude in the datablock or a primary target becuase he was so low. Another day, the same Tracon sector wanted to do a manual handoff on a guy over 50mi from my boundary who wasn't even within my scope range when they called.
To be fair [to the Tracons], there are people in my Z that flash guys 100mi (and 15min) from the boundary, and others who can't even wait for the airplane to leave my airspace before they flash to the next controller, whose boundary is 80mi away from mine.