Retrofitting a garage floor drain system with in-floor heat

[CurlyDave]

I don't know how time consuming it would be, but I bet you could find the in-floor heating tubes with nothing more exotic than a IR thermometer.

Look here: Infrared Thermometer with Laser Targeting, Non-Contact

Use the thermometer to find a tube and trace its entire length out. Using a straight edge draw a pencil line on the floor. Keep finding them. A pattern will soon emerge.

 

[Redneck in training]

Have you thought about pouring a little more concrete, sloping to the center of each bay and toward the door. That would be the safest. A 1/2 inch would be plenty of slope.

That is what I would do.