There is a relatively new user, who has asked a number of questions to which I have commented. The questions have disappeared, and they are not on his/her activity profile.
The reason I have noticed this is because I became interested in the subject and had some links in comments to the disappeared questions which I can no longer find.
How does this happen? The user deletes the questions and then they disappear from the profile? As all the questions are about the same subject, it defeats the purpose of the site, as there can be no buildup of knowledge if the user deletes questions he/she is no longer interested in.