Here is a story from Minneapolis about the destruction of police records some of them pertaining to confidential informants. In one case the records were destroyed by police and in the other as a result of public disorder.
Securing documentation - Intelligence Work 101
A distressing story from West Midlands UK about the loss of confidential intelligence on gang members. This article discusses why losing confidential material of this type, should not happen. We are in the 21st Century and there are secure encrypted methods for keeping intelligence safely and a handwritten police notebook is not one of them.