To find a user who is logged out, you can use the Upscope support code to find their screen. You'll have to activate this by selecting the option "Enable phone support lookup code?" in your 'General settings.' All the customer has to do is hover over the button on the website to display the code.
You can also enable the feature where all the customer has to do is press 'Control' 5 times on their keyboard to bring up a 4-digit code.