Hmm, yeah, it's a strange problem... googling it brings up a wide variety of results.
The other asset is Water2D Tool, and the file is water2D_Icon.png (another icon, strangely enough). Not sure where the icon is used, but it's probably one of the gizmos.
Were you able to reproduce the bug when the other person reported it? Looking at this thread, it might be affected by the project's folder layout in some way (like Editor vs non-Editor folders) https://forum.unity.com/threads/an-asset-is-marked-with-hideflags-dontsave-but-i...
I'll delete the meta files for the icons and see if that helps. Wish there was an easier way to reproduce it, but it seems very sporadic for me.