While flying around in a Creative copy of my world, I heard a zombie villager where there shouldn't be one. After a long search I finally discovered it in the top of an oak tree. (See screenshot.) It was completely surrounded by Leaves, which is why I couldn't find it until I followed its moans. The only air block was the one its head was in. (Note that in the screenshot I had broken some of the adjacent leaf blocks above and beside it, to check whether it was in fact standing on Leaves.)

The conditions for spawning a Zombie Villager (or most hostile mobs in the Overworld) include that it be on a solid block with no obstructions in the 2 blocks above it. This example appears to have violated both of those conditions.