This bug was mentioned in Watiggi's BtS bug thread, but I would like to make sure it gets noticed:

*I do not have the game yet, but Watiggi confirmed this for me*

When a privateer of an unknown civilization reaches your borders, you will be introduced to the unknown civ, and able to chat with them. This effectively gives away the nationality of the privateer unit.

This will be a minor annoyance in single player, but could have larger consequences in multiplayer when privateers need to stay private.

Bug Description:
A privateer from an unmet civilization will count as "meeting" that civilization for diplomatic & trade purposes.

Expected Behavior
Privateers (and other "hidden nationality" units should remain hidden nationality and should not trigger diplomatic relations.

Actual Behavior
Seeing a hidden nationality unit results in diplomatic relations / meeting the owner civ of that unit.

Steps to reproduce

Download the attached zip and extract the save file.
Load the save and end the turn.
Observe that a privateer will sail into view. The privateer is 'hidden nationality', but actually owned by Augustus Caesar.
Observe that Augustus Caesar will now pop-up to "meet" the player. Confirm that the player now has full diplomatic contact with Caesar.