As a Grandparent to 2 16 year old boys, who both have 15 year old girlfriends, I feel reasonably qualified to comment.
Honestly, they probably just want to hang out together. YOU are the one with the problem and issue, by 'presuming' they will sneak about while you are asleep. Generally, they will watch TV, eat some snacks, scroll on their phones, and sleep.
The negativity from you Grans is really horrible to read. You sound like a bunch of old-fashioned judgemental people, with pursed lips, acting like your own parents. Times have changed, kids don't need to sneak around any longer, risking the wrath of their parents in the 1960's.
And so what if they did cuddle up on the sofa, do you REALLY imagine they are going to horror of horrors, have sex with their moralistic Grandparents in the house? I don't think so.