In this list you can view the lots that are not registered or not completed.
The one that dissapeared was probably marked by the sender as "Mailed". It had a message like "Auto registering in ...". If you would had not maked it as "Not received" the lot would have been auto registered. This auto actions are for the situations where one of the members in no longer active in order to send/register the lots. In this case, you both, marked the lot as "Mailed" and "Not received", so this lot is now considered lost. No more auto actions will be taken on this lot, therefore is not in the list anymore. But you can still view it in the page "Stamps exchanges", section "Expired".