Any suggestions besides the obvious pencil or highlighting with colours? I am stumped, this was trickier than I thought. It is for a search function/filter if that helps but a universal symbol would be prefered.
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Does it need to be an icon? A number of the editors I use show an asterisk next to the file name in the title bar to indicate that the file has been changed.
What are the things you need to show that have changed? I might make something related to that. Something truly universal can be difficult; I've seen people use a triangle to symbolize a "delta" but that is too mathematical for most people to get.
Whatever icon you select is likely to be specific to a given culture, so consistency is likely to be more important than the actual icon.
Things I've seen used ...
A tiny (8x8 pixel) disk shown next to the top right of the field
Colour and Style change - modified fields changed to Bold and Blue
As a side thought: I think highlighting modified fields is a good idea.
Maybe a pencil erasing a sheet of paper?