Hello David,
Purchase Requisitions will be in the following tables when they are created.
POP10200 Purchasing Requisition Work
POP10210 Purchasing Requisition Line
Then after they have been Purchased (converted to a Purchase Order) they move to history into these tables.
POP30200 Purchasing Requisition History
POP30210 Purchasing Requisition Line History
I would recommend checking these tables for your Purchase Requisition in question to see if there is any missing data.
Beside a user intentionally removing history, there is always a chance that a user lost connection to the SQL server when posting the requisition so it could not properly move to history. Unless you have some sort of Audit Trail product in place there would not be a method for determining who removed the history. Any user who has access to the following window could have done it. (Purchasing >> Utilities >> Remove Purchasing History) Removing security to that window for users that should not have access to it would be a good idea.
Thanks,
Isaac Olson
Microsoft Support