The STO signal in Listener or Watchmanager means the device has detected that a Stream has timed out,
(Stream Timed Out - STO ) meaning the Avlytics device has lost connection to the url for this Channel. 

This usually occurs if the DVR details have changed, Ie. Username or Password changed, or IP or port has changed. 

After a Device sends an STO it should auto recover if it was a simple network issue that can auto resolve. 

When it Recovers from STO it will send a STR ( Stream Timeout Restored ) Signal.

There are some Cases where a Device will send STR's and the customer with Query why they didn't get STO's but did get STR's . This is typically observed when the Avlytics Device goes off or offline .

If the Avlytics Device goes offline then it cannot send any signals , which means you wont get any STO signals.
As soon as the Device recovers it could send STR's as the Streams would be Reestablished.

So if STR's are Received without previously receiving STO , there is a good chance that the Device was offline due to a Power Failure.