There isn't?
Actually, there just about has to be.
Most "new" GFCIs have a LED that only lights when the device has "tripped." The power for that LED has to come from someplace.
The analog of the "differential current" is a very weak signal. It is "amplified" to provide the power needed to "trip" the contacts. There is a tiny electro-magnetic which is activated.
In utility applications the difference current is measured in amps or hundreds of amps and this can provide plenty of energy to "trip" something. But a home GFCI is expected to "trip" of a few ma of net ground current. It just needs "amplification!"
Funny thing you post this link.
The last picture is a schematic of a "typical" GFCI. It clearly shows the electronics being powered from the "line" side. No "line" power and no electronics power.
The GFCI is a good device; you can't force it to power a fault by holding down the "reset".
It protects against a lot. But it doesn't protect (I think) from a disconnection of the neutral while the GFCI is in a "un-tripped" state.