"When using a user-data shortcode, the message "Cannot find specified user." is rendered when a logged out visitor is viewing the page. I know that user information should be wrapped in further conditionals to display only to logged in users but a lot of regular users may not give thought to that. Displaying an empty string might be a better fallback for logged out users."
Here's another quote from @EngageWP:
"When using a user-data shortcode, the message "Cannot find specified user." is rendered when a logged out visitor is viewing the page. I know that user information should be wrapped in further conditionals to display only to logged in users but a lot of regular users may not give thought to that. Displaying an empty string might be a better fallback for logged out users."