There are a few possibilities:
The garage floor drain could be connected directly to the city sewer main which, considering current building code, would require to have a grease trap installed (to trap petroleum containing liquids)
It might be tied into an ejector or sump pump well (illegal without the grease trap)
There might some kind of a French drain under the concrete slab or next to your house where this garage floor drain discharges into. One of the ways to create such drain was to place a metal drum filled with stones directly beneath the floor drain opening.
You could rent a snake camera from your local home improvement store (Home Depot or Lowes may have it) or hire a plumber equipped with such device.
If you have a sump pump / ejector pump pit with underground drain pipes connected to it, insert a garden hose into the garage floor drain and check if there is any water discharging into the pit.
Another method is to use an infrared camera (you may be able to rent one as well). Pour hot water into the garage floor drain, depending on the floor thickness you may need to keep pouring it for a few minutes, and use the camera to spot warmer areas beneath the concrete slab.
Let me know if this helped,