Whenever I'm tooling around in the Citadel, I find that I can overhear conversations between other residents of the structure. If I listen for long enough, or I return to the same group of people often enough, I sometimes get a journal entry for some new thing I should recover or area to explore.
Are any of these ambient conversations missable? Should I be cautious when scouring the Citadel (or any other areas, such as the Normandy?) so as to find them all before I advance the plot?