I see your point, I reread the reference and saw adverbs like suddenly, frequently, etc, in the beggining of the sentence are followed by a comma, otherwise it would lead to confusion by the reader. The example given was with "frequently", though.
I also agree that many authors are in the trend of minimal punctuation, so even where they used a comma (after an introductory "suddenly" for example, some don´t use it anymore. And yes, it seems it gives a new flow to the thought and reading.Suddenly, there came a clap of thunder.
Suddenly there came a clap of thunder.http://www.gsbe.co.uk/grammar-commas-an ... stops.html
Indeed the subject of isolatin commas is a bit of a puzzle, not even them, the grammarians don´t agree much on it. You can read on the above reference site (under the head pausing) that they may use commas to indicate speech pauses, even if goes against the grammar grain.
To top it all, one could claim that the bit "when suddenly" it´s a parenthetical information, so commas would function as a parenthesis to insert that information and the author judgement as the thing happened.
Here, in a text we can see "Imagine children growing-up with their mother and father for the first few years of their life, seeing
Them daily and forming habits with both parents, when suddenly, one or both parents leave."http://www.skylinecollege.edu/skyenglish/complete.pdf
So, it has more to it than the eyes meet, but I like the notion of putting commas where the intonation changes naturally.
Back to the initial question, by this last author trend the sentence of our question would morph into "We were walking at the airport lobby in direction of the metal detector, when suddenly, an elderly man talked to me..."
Anyway, it´s up to the writer, I think. It´s open to more interpretations and comments.