There were three main lines of retreat, but I couldn't trace exactly where they were coming and going to, as thousands were just running around wildly and I couldn't get close enough to track what was going on without crushing them (or getting ants running wildly all over me.) One appeared to be heading somewhere under a stump on the patio, and another into a crack, but I fear that some may have taken up residence in the bag of potting soil. I'll have to gently discourage them if that's so.
I quite like ants, as long as they stay out of my house. I wonder, will they split into little colonies wherever they managed to evacuate some eggs to, or will they eventually reconnect and ingather?