Ocifant,
I may be wrong but can think of no reason why fieldnotes would have been culled, particularly if the sites remained listed. It is certainly not something the Eds routinely engage in. In the past some posts for holy wells have been deleted, but if you really have lost fieldnotes on obviously prehistoric sites (I'm assuming this was some years ago now) I can only imagine it to be a technical hitch (there was an issue with how they linked to the weblogs at one time).
TMA Ed.