I suppose I have based it on an assumption that due the process of click and send or ebay labels - they can only be processed when goods are marked unsent, and the system marks it as it as sent.  This doesn't account for post I have received processed in other ways though.

I will watch out for the next batch of marked as sent advice emails and retain them to check for clues as to the type of parcel actually being sent.

I think the tracking number one is a quite a good development - but would like to know what triggers it.