NOAD has a dictionary entry for high note, saying:
high note (n.) a successful point in an event or period of time : he wants to end his managerial career on a high note.
In addition to high notes, there can be low notes, bad notes, ominous notes, and notes of uncertainty. Under its entry for note, NOAD says:
note (n.) a particular quality or tone that reflects or expresses a mood or attitude : there was a note of scorn in her voice | the decade ended on an optimistic note.
So the headline you are quoting indicates that the mood at the end of the talks was one of uncertainty – everyone is unsure of whether or not any good will come out of the talks.
This phrasing can be used in a variety of contexts; it seems to be often used to describe the overall mood at the beginning or end of some event.