What is the meaning of a red exclamation mark icon in Calc's status bar?
There is no information displayed when I mouse over the icon, and I can't find anything about it in Calc or here.
If I click on the icon it disappears.
The icon indicates whether the document has been changed since last saved.
Clicking the icon saves the document.
Related:
Hello @Pheeble,
This icon shows the Document Saving Status ( Unchanged – Unsaved – Saved ).
If your document has unsaved changes, then an extra visible mark is displayed inside this icon. Clicking on the icon will then save the document.
In my case, using the Icon Theme “Galaxy”, the visible mark for unsaved documents is a red "*" mark: 
.
Other Icon Themes might use a different mark there.
Although i have not seen this exclamation mark inside my Document Saving Status icon before, i suspect that it is your Theme’s version of the doc_modified_yes_14.png mentioned above.
Confirmed: No Tooltip is shown for the Document Saving Status icon when pointing the mouse to it ( reproduced on my LO Version: 5.4.2.2 Build ID: 1:5.4.2-0ubuntu0.17.10.1 ).
HTH, lib
Asked: 2018-01-20 22:46:43 +0100
Seen: 137 times
Last updated: Jan 21 '18
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What version? The images in my answer were captured from 5.4.3.2 on Windows.