From February of 2008 up to Late November of 2016, Danbooru rewrote select imageboard sources to "Image board" on upload, such as those from 4chan, Cynthia and Yande.re. Due to the nature of things, it is unlikely for those automatic changes to be reversed. (forum #122820)
As a result, post with such sources have automatically received the non-web source tag, due to their now-textual nature.
This tag is for such posts (as well as those posts that were manually desourced from their original by users uploading). If possible, the sources should be standardized for ease of access (forum #152518), if not restored if in any way possible. Posts which have had their sources changed to either their original first-party source, a different text source, or to nothing cannot reasonably be accounted for.
If the source is changed to the image's original first-party source, assuming the source in question is still active, and the md5/EXIF data might not match, please add replaceme to the post. For first-party sources that aren't available anymore (bad id), check tools such as SauceNAO and ascii2d, as to see if a link to them can still be obtained (though in such a case, this tag could arguably be retained on the post in question, due to how imageboards can potentially change that data).