﴿ 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,”

Please check ayah 28:13 for complete tafsir.