﴿ وَقَالَتْ لِأُخْتِهِۦ قُصِّيهِ ۖ فَبَصُرَتْ بِهِۦ عَن جُنُبٍۢ وَهُمْ لَا يَشْعُرُونَ ﴾

“And so she said to his sister, “Follow him” - and [the girl] watched him from afar, while they [who had taken him in] were not aware of it,”

And she said to his sister Mary ‘Follow him’ in other words follow where he goes in order to find out his news. So she watched him from afar from a distance secretly while they were not aware that she was his sister or that she was even watching him.