If I connect to one cluster (have tries local and atlas) and then disconnect in compass (menu bar >> connect >> disconnect) and then connect to a different cluster, any databases that have the same name and any of the same collections are still connected to the first cluster.

Here's an example.  I have an atlas cluster when my production db's live.  I also have a local mongod running on my localhost, I have some dbs and collections that are named the same on both servers.  Sometimes I pull production data to my localhost to test.  If I connect to my atlas cluster in compass, and then disconnect and connect to my localhost in compass without closing compass, the dbs and collections in my localhost that are named the same as in atlas, will still be connected to atlas and not my localhost.

So when I think I am connected to my localhost, I am actually connected to my production atlas, but only for the dbs and collections that are named the same.

See screenshot below, where compass is connected to LocalHost, but DB is pointing to Atlas Cluster.