There is usually an audible "whoosh" that accompanies a Laethan using Air magic to fly, the sound diminishing to a whistling at very edge of hearing when the spellcaster is simply allowing him or herself to hover. Between the above-average hearing common to his Order and his familiarity with those exact sounds (being a member of the Order of Air himself), Avry'l was doubtless expecting to be able to detect any flying pursuit by sound alone, and so was not looking up.
However, some Laethans take their flying more seriously than others. A good competitive flier can dredge up, without looking at a single book, anywhere from seven to twelve modifications that can be made to any spell of flying, not counting his or her own personal secret spell formula. A particular pair of the aforementioned snippets, used in tandem, can reshape the pressure field around the flier to reduce drag in a race; this combination dampens the flying sound, as well. So girded, a flying Laethan can take to the sky and not make a bit of noise unless he or she is pushing the spell to the limit in a sprint.
Wind Guide Jagen Turlis was considered a promising young talent among competitive fliers, a "wing to be watched", as the lingo went. The fact that a significant chunk of his time in the air was spent in the near-frictionless cocoon of a competition-level flying spell meant that the basic spell of flying was, to Jagen, banal at best. So when Jagen took to the air on New Years' Eve, he did so in dead silence and with all sorts of little additions to the experience of flying.
So when Jagen caught sight of movement on a roof on one side of the Stone Road and shed altitude to investigate, the man on the roof didn't notice him until the two were in easy speaking distance. It took a similar stretch of time for Jagen to make out the facial features of the man - features he knew all too well.
"Out for a stroll, Mr. Vancougar?" Jagen asked from about three feet above eye level, intentionally and conspicuously omitting both Avry'l's proper title of "Wind Guide" and the respectful use of a fellow soldier's first name, or, possibly, denying the veracity of Avry'l's claim as a Laethan altogether by calling him precisely what would be polite to call him were he a layman.