Well, last year I had the engine out to do the chain guides and I decided replace all the rubber and gaskets I could find other than the head gasket. Although I though I covered everything for some reason it looks like I decided to not replace one coolant block gasket in the back of the engine. Just a coincidence, this week I took the tranny out to swap it with a rebuilt one and found that gasket leaks. The leak started a week before that. So, I haven't found enybody so far that mentioned that gasket needs to be replaced but in my case it looks like it failed. Now, just an additional information, I am using evans NPG+ low pressure coolant which shouldn't be the cause of this but just in case.
My X is a 2000 with 230k KM 4.4L. So,you might want to consider replacing that gasket if you have the tranny out for other reasons.
Removing the transmission is not a fun job at all.