When a customer has the Hidden Contact option enabled their contact information and sales history will be hidden throughout the software.
At POS only the customer ID and name will appear and the Contacts button will be disabled.

Similarly, the receipt will only show the ID and name.

On the customer record, the Contact Methods will appear blank.

The same is true for the Activity and Merchandise tabs.

And when viewing a list of customers the text CONTACTS HIDDEN will appear in place of the contact details.

To unhide the details on the customer record click Supervisor > Show Hidden Contacts and Activity.