That's very odd, what Unity version are you using?
instead of a warning symbol it should be showing the open symbol. In the code I'm using
Code:
UnityEditor.EditorGUIUtility.isProSkin ? "open-dark-icon" : "open-light-icon"
And it looks like this:
View attachment 12806
My only guess is that the icon is not found and a warning icon is provided by Unity instead.
Perhaps it changed name in your Unity version, or there is something wrong with that version at it will be fixed once stable. I'm not sure.
In any case it can be ignored