Version store blowing up, but responsible sessions have no open transactions
This issue was caused by an orphaned distributed transaction.  Since the transaction is orphaned, the session ID is invalid by the time you see it.  To clear the condition you find the oldest transaction with an active snapshot, lookup of the unit of work (UOW), and then kill the UOW.

--find the transaction with the oldest snapshot version
select top 1 transaction_id, elapsed_time_seconds 
from sys.dm_tran_active_snapshot_database_transactions a
order by a.elapsed_time_seconds desc

--using the transaction ID from the query above, find it in the active
--transactions DMV.  You should see the begin time as many minutes/hours ago
select transaction_id, transaction_begin_time, transaction_uow 
from sys.dm_tran_active_transactions where transaction_id in(&lt;transaction_id&gt;)

--using the transaction_uow from the query above, kill the unit of work 
kill '&lt;transaction_uow&gt;'


After it is killed, run the first query again to see if there are any more that need to be cleared.

In some cases you may need to find the UOW by first finding orphaned transactions, which are supposed to have a session ID of -2.

select * from sys.dm_tran_locks where request_session_id = -2


See Kill (Transact-SQL) for more.