The narrative is that while frozen in fear, they decide to quietly keep going at it anyway.
It's Eric walking by outside, which wouldn't really have the same impact in the Samantha version, but since Samantha's version isn't canon anyway we can just imagine that it's her fiancé Greg walking by outside (or something).
For the final shot (where they resume fucking), I may do a lil' something to give it some oomf or velocity, but I'll see how the text turns out first. In general, the camping scenes are very much going to be about silent, tense sex instead of the usual screaming match.
I'll have some more status reports on the camping update soon.