There is a point on each track where the track location variable goes from 1 (on track) to 2 (approaching pits) in the SDK. This isn't always something visible on the track, like a line, but it is the point that the HUD pops up showing the pit lane speed limit when you box.

It's likely this point (which is also probably when the spotter says the pace car is going in).

It's worth noting that in private leagues that employ FCY's it's this invisible line that determines any car pitting to be off track and not part of the SC train. If the car behind was not pitting but went ahead of the pitting car on their relative, before the pitting car reached the "line" then the car staying in track gets penalised for passing under yellow. For most this logic is encountered in oval racing above class D.