There is nothing better than receiving a thank you note for simply doing your job. I must admit that I have never thought of it in the context of the writer being a potential top talent though. A thank you note certainly demonstrates a number of valuable behaviours including gratitude, thoughtfulness, proactivity and resourcefulness all of which would be good to haves for a values based organisation.
It represents a decent person which is a good starting point but I think there is a lot more to do to qualify if the candidates is truly a top talent.
Here is another article that looks at the behaviours of a high potential leader http://thoughtleadership.collingwoodsearch.co.uk/post/102epls/leadership-potential-what-employers-are-looking-for-its-not-one-size-fits-all
My team and I have spent years establishing a world-class hiring process at Acceleration Partners (AP), but for all those efforts, I've found that one of the strongest predictors of success is still something right out of my grandmother's playbook: A thoughtful, well-written thank-you note. Thank-you notes can tell you a lot about a job candidate. Here are the four qualities that stand out most to me. Gratitude: People who show gratitude think about others and not just themselves--a valuable quality at any workplace. A 2011 University of Kentucky study found that people who are grateful have more sensitivity and empathy--attributes that tend to make people better leaders and team players.