It's possible that the charging spot was ICEd when the blue S arrived, so he's charging from the spot next to it. Then the 3 arrived and installed his adapter in the hopes that the S driver would plug him in when he's finished. Ya never know.
I'll agree. For the non-Tesla crowd, if you arrive at a charger, and it is in use, then if you open your charge port, that's a signal to the currently charging user, that "When finished, please plug me in"
On my Leaf, I can open the charge port and it stays open, doesn't work that way on the Tesla, but by shoving the charge adapter in, it is the effective equivalent.