Not sure if this is how this works but:
Here, Belkar notes he failed a spot check, which is a dice roll in D&D 3.5 that determines whether or not a character sees something. It is one of the more common rolls players (or their DMs) make during a gaming session, and often if a player rolls (or hears rolled) dice when entering a room or traveling, but nothing happens after the roll, there is a sense that you "missed" something, even though the character should not realize a check was failed.
Alertness is a feat that provides bonuses to listen and spot checks. In 3.5, so long as your familiar was within reach, you gained the benefits of this feat. although meant to be magical assistants/companions for wizards, players would often forget they were around, and so they would "pop up" whenever their abilities were needed, thus giving players a reason to describe their location/activity.
A listen check is identical to a spot check, except that it represents the ability to notice a sound, rather than to see something. It creates the same "dice rolled, but no new information" issue that spot checks create. As noted above, alertness provides a bonus to listen