What is return current of crystal oscillator?
I think you have mis-interpreted the idea of return currents. This does not refer to the currents flowing in, around and through the oscillator but to the other devices 'sharing' the ground track and using it to 'return current'.



C1 and C2, together with the input and output capacitances of the gate form the capacitive load of the crystal and so part of the total oscillator circuit. {see http://www.mpdigest.com/issue/Articles/2008/Mar/Crystek/  for a fuller treatment}

What the hint suggests is that when connecting the capacitors you keep the leads short (avoiding any parasitic inductance/capacitances - we are dealing with a few pF here) and avoid the return currents of ... The easiest way to avoid return currents is to connect the two ends of the capacitors directly. (see below)



As you can see there is no DC path through the crystal or to ground. Impedances are very high so even small currents can have a significant effect. By keeping the load capacitor leads short (and essentially connected at a single point) we avoid other currents flowing along the (same) ground return track between the capacitors and allowing the currents to mix. Hence the the hint 'avoiding the return currents from USB, RS232, LIN, PWM......and power lines". ( for further information check out ground loops)