Those outlets merely they work by making sure the hot current and the return current are the same. If they are not you have a sneak path somewhere in the house.
One such place is with fluorescent lighting. i've heard of several instances where a sneak path to chassis (greenie) through the magnetics in the ballasts has happened.
Check your meter, if you do have a sneak path your bills are probably abnormally high and the meter is spinning like a record.
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