Double-check that none of your four NAV radio frequencies are tuned to any vessel attached to the UCD. [A simple way to check is to bring up your COM/NAV MFD and see if any of the four NAV radios have text below the frequency like "XPDR foo" or "IDS D-01 foo".] If any of your four NAV radios are locked onto a vessel, the XR checks whether the closest vessel is within 100 meters of the docking port, and if so, whether the distance from the docking port to that vessel has decreased since the previous timestep ("frame" in the simulation). If both conditions are true, the warning is played.
If you are getting the warning all the time and there are no other vessels in range besides those attached to UCD, what you need to do is to retune the NAV radio that is locked onto a vessel attached to the UCD to some other frequency (so the ship won't be locked onto that vessel anymore). It is odd that the warning keeps playing for a vessel in the bay at all, however, because in theory the distance should remain exactly the same at each timestep. However, perhaps the distance is varying in the least-significant decimal place every few frames or something due to a rounding error, just enough to be detected.
So to sum up: the fix is to retune your NAV radios so they aren't locked onto any bay vessels. :thumbup:
