It's called inherent sideslip and exists in all helos with a TR.
The TR (or the fin, or vertical stabs or combination thereof) has to always produce anti-torque force and so, with the 'wings' level, you will be slipping slightly sideways. See Shawn Coyle's book for details.
Think about TR drift in the hover and how that is corrected with lateral cyclic - then extend that principle into forward flight.