Return Current Through Split Planes (1.5V, 3.3V, 5V)
For high frequency signals the return current for that signal is on the adjacent plane. It tends to be directly under the signal path. The value of the plane does not matter. So long as the plane is at a fixed voltage.

If your plane is ground, then the return path would follow the signal path (back to your driver), and end there.

If your plane is a power plane, then your return path would follow the signal path and then find its way back to ground through the past of least impedance. If you have a decoupling capacitor nearby, it will pass through the capacitor. Otherwise it will find its own way and may start interfering with other signals until it makes it way back to ground.

If you have a split plane, the return path will follow up to the split, and then it will find a way to get across. If there is no immediate least impedance path available, then it will fringe across.  Fringe lines are not straight, and will spread out, and again, potentially interfere with adjacent signals. 

If you have to use split planes, throw a capacitor on each side of the signal, so that you can establish a return path, through the capacitor. However, with capacitors internal resistances and inductance, you may see some noise on adjacent signals, or ringing on your signal. However this is better than not having the caps there. Ideally, you would route your signals such that they don't cross split planes.