Okitegami Kyouko no Suisenbun

The second instalment of the Boukyaku Tantei Series.

A young man, Oyagiri Mamoru, works as a security guard at a museum.

Whenever he visits the area he guards, there's always a beautiful white-haired woman who stops in front of a particular painting. She is Okitegami Kyouko, also known as the Forgetful Detective. They were only supposed to be a guard and spectator until a number of tough cases involving art occurred—.

(Source: Kodansha, translated)