As I found this out today and never been really stated by Virtrium I figured I would quote what has been said to me in a support ticket.
I asked in a ticket why I was missing items from my previous plot as the reclamation process should be returning the items to your vault. At least that is what I - and probably most players - were expecting.
This is the answer I got;
Items are returned to characters based on who last owned them, not on what plot they were stored on. If someone else had either made use of an item, or put it into their inventory to "move it" around on the plot, then the item would have been owned by them. There would have been a period of time between when your account was inactive and when the plot was reclaimed that it's possible guild mates, or anyone else who had access depending on how the structures were set, could have used the item and thus it would have been sent to them.
So remember, not all items are guaranteed to be returned to your vault!
I hope you get all your important items back!