Jennifer had no idea she was being written out. She went to work as normal the day production was to begin on The Gift and on her script there was a post-it note from Rick Berman that said something like "We'll miss you."
I find this incredibly difficult to believe.
Jen has said that her contract was not renewed. She would have known that long before she showed up for filming.
And not being invited to the up-fronts with the rest of the cast would have been another clue.