Check if a Unit is Inside Foreign Borders

[Dr. Aardvark]

I'm currently working on a new leader and agenda. I want the leader to heavily dislike when the player moves civilian units that can enter foreign lands (missionaries / apostles / archaeologists) into their borders without first making an open borders agreement. This would require checking for correct unit type, the absence of an open borders agreement, and whether or not appropriate units are in the leader's territory.

Unfortunately, there doesn't seem to be any sort of requirement that checks for a unit being in foreign territory. There's a requirement for a unit being in enemy territory--but as far as I can tell, that's only valid when two civs are at war with each other, which isn't what I need. There also seems to be a requirement for being near a civ's borders, but using that would probably set off the leader too quickly.

I considered checking for a player spreading their religion or digging up artifacts instead of just simply having units present. However, while there seems to be a requirement for spreading religion, I can't find one for digging up artifacts.

I just can't think of a solution or workaround given the available list of requirements. Is there any possible way to accomplish this task?