When connected to a replica set a background thread should monitor the replica set by polling isMaster for each member. If a secondary goes offline it should be removed from the list of secondaries. When a secondary comes back online (or a new member is added to the replica set) it should be automatically detected and added to the list of secondaries without having to call MongoServer.Reconnect.

Not that in a heavily loaded environment failed secondaries will almost certainly be detected during a regular operation, as they are many times more frequent than the background thread polling. However, the background thread is the only way that returning or newly added secondaries will get added back to the list of secondaries.