If you dropped af:table inside af:panelCollection, then by default the buttons 'Datach' button icon will be rendered. Similarly, if you specify columnSelction="multiple", the buttons 'Wrap' and 'Freeze' will be rendered by default. So, if you want to hide those buttons, just specify featuresOff="detach freeze wrap" for af:panelCollection.
Sample code below:
<af:panelCollection id="pc1" styleClass="AFStretchWidth"
featuresOff="detach freeze wrap">
PanelCollection without "featuresOff" set:
After setting featuresOff="detach freeze wrap":
As you can see, the panelCollection is clean without 'Detach', 'Freeze' and 'Wrap' icons after setting featuresOff="detach freeze wrap" .