A friend of mine has a feral swarm in her garden, it had settled before she noticed it, and as it was in a very difficult position to retrieve, she left it. It has survived at least one winter, and seems to be attracting swarms to her garden. The latest one she was able to retrieve, and it seems to be settling nicely.
A swarm settling over the weekend as you describe, could be at risk of being disturbed when the heavy traffic resumes, and anyway any bee-keeper able to retrieve it would be very grateful for the tip off. The price of colonies has gone through the roof lately on account of the massive die offs over the last few years.