Should I let my resident just cat lay infront of the new cat's door

I'm doing a slow introduction with my 2 cats. They both will eat up against either side of the door from eachother. However, often times, my resident cat will go up to his door, lay/sit down and just stare at it. She hissed at the new cat a few days ago, but usually she doesnt.

Is this just curiosity or guarding? If it is guarding, is this a negative association that I should be discouraging or should I just let my resident cat do her thing?