A couple ideas for you to try:
Esri told me this once:
If you think you have an orphaned replica version then the way to find
out which one it is is to synchronize all replicas and then delete them
in ArcCatalog, then compress.
Next check the SDE.Versions table in SQL Server. If there are any
versions called something like sync_send_xxx then these are orphans. To
delete these orphans change the status flag from 16777216 to 33554433.
This will make the version visible in ArcCatalog version manager. From there you can delete it and run a compress again.
Also here is a sql script to find replicas:
-- put in database name
use databasename ;
select i.objectid, i.name from sde.GDB_ITEMS i join dbo.GDB_ITEMTYPES it on it.uuid = i.type where it.name = 'Replica' order by 1 ;
-- compare results above with the following
select v.name, v.state_id from sde.sde_versions v order by v.name ;
--
Hope this leads you in the right direction.