[While she doesn't think her clothes do much in the protection area, she still ventures outside the barrier to check the situation. The monsters are weak so Pearl certainly doesn't take long at all to get rid of them. That's better, really: Steven is one of those in charge of taking care of the tree, so she just had to make sure not a single one of those things got close to it.
It's just when she is getting back inside the barrier that she spots Sansa being so close to the border.]