Ask Melissa to go to the restroom with you

You lean over to Melissa and ask her softly, "Hey, do you think we could make a quick visit to the restroom?"

Melissa nods, "Sure... someone might take our seats while we're gone, although we could sit somewhere else too of course."

You're not too keen on the idea of having to sit with complete strangers if someone does end up taking your seats, but you do have to go pretty desperately.