From: Rabbi Moshe Sofer
To: Rabbi Yaakov
A woman who did a casual bedikah before going to synagogue later found a ketem within three days of her hefsek taharah. The Chasam Sofer rules leniently, arguing that even an informal examination carries Torah-level validity as a pekidah since the woman had some intention to examine herself, and since any remaining concern is only Rabbinic, the ketem may be attributed to the external cause.
Source: Shu"t Chasam Sofer YD §187