I've seen the following ground symbol in quite a few schematics:
How is this different from the typical GND symbol?
This symbol is frequently used to indicate chassis ground, i.e. ground tied to the chassis or enclosure of the device.
This is distinct from, but may be connected to, analog, digital or power ground (circuit ground) depending on the circuit configuration.
The Chassis Ground may or may not be connected to the Earth ground of a building, i.e. the mains power ground line.