I don’t understand why wariness, abstract and uncountable, did take article “a” before it?
- there was a wariness in her tone.
The indefinite article can be used to refer to slight quality or tendency, something just beginning to show itself or showing itself only slightly, or to a quality which is difficult to name or pin down, either because it is a mixture of things or ephemeral in nature.
There was an excitement in her voice.
She was not bubbling over with excitement, but a degree of excitement was present and detectable.
There was a certain something in her voice, but I can't say just what it was.