There are 2 ways to hide a sheet.

Most people know the first way:
Format => Sheet => hide

Not many know the second way:

Press ALT + F11 to bring up the VBA editor.

.. and change the visible option as shown:

To the untrained eye, this Very Hidden sheet is difficult to find.

If you don't see the "properties" box, single click on "Sheet2" and press F4

Training Video on how to hide sheets: