I'd be trying the dye method on the center drain to see if it's leaking around the perimeter ring, if not, it could be in the drain pipe under your pool.
One way to eliminate the drain from the possible leaks is to lay a large (3'x3') piece of old liner or plastic or anything that will seal it off, over it and weigh it down all over, this should stop or almost stop any water going into the drain, then give it a few hours to see if the leak outside the pool subsides or stops.
If it doesn't you likely have a leak in a different location - first place I'd be looking is near the stairs, then the return/skimmer area, those are the primary places then, it could be anywhere else in the pool.

Another way to see is if it's only leaking into your ditch in one area or if you're getting leakage all around the pool, a wall leak is usually seen in/around the same area outside of the pool, floor leaks are more dispersed and can appear whereever the path of least resistance may take it.