Very high transactions per second Our production server runs on average 4,000 transactions per second.  In the past few days the average has jumped to 175,000 transactions per second.  That is not a typo, it's 175K per sec.
Looking at the DMVs for transactions, we can't link it to user sessions directly, but we do see this:

SELECT NAME,
       COUNT(*)
FROM   sys.dm_tran_active_transactions
GROUP  BY NAME
ORDER  BY 2 DESC 


--

+------------------------------+-------+
|             Name             | Count |
+------------------------------+-------+
| WorkFileGroup_fake_worktable |   627 |
| LobStorageProviderSession    |   217 |
| workfile                     |   171 |
+------------------------------+-------+


Can anyone shed light on these types of transactions?  Or am I chasing ghosts here?
Watch for the high activity again; when you see it, start a server side trace or if necessary use Profiler briefly to see what's going on.

Alternately, use a packet sniffer like Wireshark to capture raw wire activity.

Check dm_exec_cached_plans to see if that gives any idea what's going on.

Watch dm_io_virtual_file_stats to see which files in particular, if any, are being hit.