Quick question about ground
That is competently depending on the manner you design your circuit.

For example, some circuit designs have multiple ground plane. One can be for power circuit, another for high speed signals, third for AC. Those three ground planes can be separated and have different potential when compared to each other, but within that portion of the circuit, they'll be the neutral reference point.

A "ground" point, doesn't necessarily mean neutral, zero potential. But, if the circuit you're referring to, has been connected earth ground, to the local AC circuit "ground", then yes, the current it's returned to earth ground. 

But remember, it's current, it likes to go places. Usually it prefers the faster and easier path. And your AC circuit is way closer than the ground wire in the wall that leads outside to the real ground earth (which depends on the facilities wiring).

As per acromag.com, Early developers of electrical systems theorized that the earth was an electrically neutral body, i.e. an equal number of negative and positive charges are distributed throughout the earth at any given time. Being electrically neutral, earth is considered to be at zero potential and establishes a convenient reference frame for voltage measurements. Noting that voltmeters read only the difference in potential between two points, absolute measurements can be made by using earth as a reference.

A true earth ground, as defined by the National Electrical Code, physically consists of a conductive pipe or rod driven into the earth to a minimum depth of 8 feet.

Current flows through the least resistive path. Your body can become the path of least resistance for a fault condition, which allows current to flow from AC hot back to AC neutral/earth ground. Againt, "flowing back from earth ground" statement, is a crude assomption that somewhere, the earth-ground connection is not a true earth-ground, or there is a fault somewhere in the path. This may cause a reverse injection of the current back to "somewhere" via the least resistive path (can be your body, or your circuit). Isolating your circuit correctly will prevent that.