You can make secret rolls in the chat window by simply turning off the broadcast switch.
I then normally rename the tab to PRIVATE by right-clicking and selecting Rename Tab.
Anything you type in a private tab goes only to you. That includes text and rolls. Any rolls you make from the Dice Rolls gallery or the dice tray or a dice roll object on the tabletop go to the selected tab, so if your private tab is selected, that roll is seen only by you.
In EpicTable 1.x, there's no indication in your chat display that those rolls are private, which is unnerving. In EpicTable 2.0, upcoming, the private rolls are clearly labeled as such.