Coolant leak but not from radiator

There's two ways that can help in finding the cause. The fastest is a simple pressure test. You can borrow the tool from an auto parts store that rents or borrows tools. With that you'll have to watch where the coolant is running down from.

You can also add a small bottle of dark purple dye to the coolant, then search later with a black light. You'll find that black light at auto parts stores too. The dye will show up as a bright yellow stain that you can follow back to the source. This method works really well with slow leaks when the coolant evaporates too quickly or when it makes a large area wet but you can't see it running from a specific place.