Our leak was due to a hole in the drain hose. The snap which holds it up had broken. The friction from the moving washing machine and the concrete floor resulted in a hole in the hose.
Replacing the drain hose was very easy. It took just a few minutes. Strangely, every video I watched showed the exterior metal housing being removed. That seems totally unnecessary. I simply put the machine on its side and worked from the underside. The only tool I needed was pliers to put the clamp back on the hose.
If I had known how easy the fix was, I would have done this long ago. Instead, I had quite a lot of cleanup due to the leak we ignored for quite a while. I think the lesson I've learned is to search for a fix sooner.