Agree OP action was acceptable in my book; however... and maybe I'm just on flights with good FA's, as the FA's do a head count and check if there is a difference before the door closes then they go over the pax seating after take off generally during the immediate drink service, they notice if they have an extra PAX up front. I have seen people get caught at this by the FA's when they serve the drinks (I didn't see them or know they actully jumped up illegally when they did this). So while I agree with the OP actions, on what I've seen they would have been caught anyways. Atleast on my flights.