It sounds like the TextBox is inside a UserControl. A UserControl is created by a developer, and is basically one control with any number of other controls in it. You can check to see if there are any User Controls in your project in the Project Explorer (Ctrl+R).
One way to know exactly what class the control belongs to is to open your form in design mode (Shift+F7), click on the control, and look at the Properties window (F4). The drop-down list should show the currently selected control's class name in bold, then the name of the object.
What is the control's class? If it is anything other than TextBox, then this would explain the unexpected behavior you experienced.