phone charger residual electric flow
I can understand in theory that there was some residual electric current in the charger, just enough to trigger my phone. 


Not quite right. There was electrical charge left in the device. Once you plugged in your phone the charge was able to move and moving charge is called current.


  However, I fail to understand the scale. How much residual electric charge could be left in a 1m phone cable, and how long before it dissipated?


No charge is stored in the cable. It's stored in the capacitors in the charger. The charge is given by \$ Q = CV \$ where Q is charge (coulombs, C), C is capacitance (farads, F) and V is voltage (volts, V).


  How does this play into the whole completing the circuit, as there was no real ground connection.


Circuits don't need a connection to ground (as in the Earth). If they did electrical circuits would not work in aircraft (insulated by air), cars (insulated tires) or your phone. What happend is that your phone completed the circuit between the charger positive and negative terminals and allowed current to flow around the circuit of the charger, cable and phone.