I was flying and found an uncovered mob spawner as is common in the desert. Unusually, when I dropped in, there were items on the floor and no chests to be found. I don't know if this happens with underground ones, but even so, it could mean people miss out on valuable rare items.

1st screenshot: The spawner as I found it
2nd screenshot: After I broke the sand at the far end, hoping to find a chest, but underneath was mossy cobblestone
3rd screenshot: The items I picked up