The unintentional glance mentioned in the narration means when a man glances at a woman without intention, or a previous arrangement or agreement. This type of glancing is overlooked. However, it is not permissible to elongate his glance until it becomes staring, thinking it is only the first glance, as is mentioned in another narration,
“You are allowed the first, and not another.” (Abu Daawood 2149)
If someone was to say: He continues to look and makes it longer and calls it the first glance. This is incorrect and goes against the order to turn away his gaze as he is commanded to lower his gaze, as Allaah says,
(قُل لِّلْمُؤْمِنِينَ يَغُضُّوا۟ مِنْ أَبْصَـٰرِهِمْ وَيَحْفَظُوا۟ فُرُوجَهُمْ)
“Say to the believers to lower their gaze and protect their private parts.” (AN-Noor 30)
Thus, if he continues to look and elongates his gaze and does not obscure his sight, he is not acting upon the command to lower his gaze. As for what occurs without any prior arrangement or intention, then this is overlooked and forgiven.