hey, i have no idea what kind of client side data gets passed to jagex, but i'm pretty sure they aren't watching your screens.
they have large volume of players, and i also think the volume of data is not the biggest worry; think about the processing power needed for monitoring 30k players at the time.
A classical computer wouldn't be enough for this amt. of work. Instead, they have an emphasis on hotzones (common bot areas) and they make use of filters. So a player that is new, uses weird connection (not home IP), stars walking through paths that are extremely commonplace etc. gets flagged instantly. Then it will be monitored more closely, to squeeze out whatever evidence needed for their criteria. (just making this up, but to give a rough idea). Anyway, if the players are grouped in levels of being "bot like", instead of monitoring 30k at the time,then jagex could organize their resources much more efficiently. Less suspicious group would gather less data and the opposite would apply.
I suggest reading logicfurys thread, he has a pretty good approach to how jagex anti ban team would operate.