Rashi on Genesis 38:15/רש"י על בראשית ל״ח:ט״ו

1ויחשבה לזונה AND HE THOUGHT HER TO BE AN HARLOT, because she was sitting at the cross-roads.

2כי כסתה פניה FOR SHE HAD COVERED HER FACE — so that he could not see her face and thus recognize her. A Midrashic explanation of our Rabbis is that כי כסתה פניה means BECAUSE SHE ALWAYS COVERED HER FACE: when she had stayed in her father-in-law’s house she had shown herself a modest woman, and therefore he did not suspect her (of being the woman who was sitting there for that evil purpose) (Sotah 10b).