Unhiding hidden parts

Can someone please elaborate on the meaning of (and connection between) 'removable' and 'hidden' for frame parts?

For info; I am trying to implement a simple enough idea:

1 - Buttons are not shown (presumably created but hidden) when the window is created.
2 - Buttons are then shown in response to an event.
3 - Buttons will probably then be re-hidden in response to a later event (timer).

I naively went for (removable . t) (hidden . t) in my frame parts and then used map-frame-parts expecting to be able to un-hide them, but the parts appear not to exist at all (they are not in the list traversed by map-frame-parts.

Many thanks, R.