My first thought in response to this was (1) why is the GM calling out stuff that s/he has already decided won't matter ("dungeon decoration") and (2) the player hasn't specified what s/he is looking for.
Like [MENTION=5142]Aldarc[/MENTION] I enjoy "fiction first" play, but I think that is a bit orthogonal to your example - because in Dungeon World a player who declares
I search the rubble is
closely studying a person or situation, which automatically triggers the Discern Realities move (DW p 68). And the Dungeon World rules say (p 18) that
When a player describes their character doing something that triggers a move, that move happens and its rules apply. If the move requires a roll, its description will tell you what dice to roll and how to read their results. . . . Everyone at the table should listen for when moves apply. If it’s ever unclear if a move has been triggered, everyone should work together to clarify what’s happening. Ask questions of everyone involved until everyone sees the situation the same way and then roll the dice, or don’t, as the situation requires.
So at least as I read the DW rules, there is no reason why a player who declares
I search the rubble shouldn't then pick up his/her 2d6 and roll them. If s/he gets an 11 or 12 (like the 20 in your example) then the GM
is obliged to provide a certain sort of information, as specified in the rules (eg
What here is not what it appears to be?). The pile of rubble
matters (regardless of whether the GM thought it would or wouldn't) because the player has (i) decided to pay attention to it, and (ii) succeeded at a check.
As I said, what strikes me in your example is that the player hasn't indicated what s/he is looking for, and so hasn't given the GM very much context to hang a response on. I find it easier to narrate successes (and failures) when I have some sense of what the player thinks is at stake in the situation.