I have noticed that thumbnails on Mac OS X don't seem to be generated every time I open a folder, but to be somehow "cached" instead.

I wasn't able, however, to find a folder where they clearly look to be stored.

I thought then they could be saved as some kind of metadata; i found, however, that when deleting a file (e.g. from the Desktop) and saving immediately after a file by the same name, the preview for the previous file is showed for ~ a second.

Are they stored separately from data? How and when are they invalidated (e.g. only when a file by the same name appears in the same directory)?