Several years ago I found a cluster in a park near to where I live and have noticed the Monarchs clustering there each winter ever since. I'm not talking about lots of butterflies, rather about 10 or so in this particular location.
I have also seen another cluster, in a park in a town that's about an hour's drive away.
Each Winter I have seen many butterflies there too.
It seems the Monarchs always return to their overwintering spots.