The start of the negotiation process was formally acknowledged in various diplomatic venues, as it usually is for these treaties: for such large negotiations the bureaucracy is huge (the days of Molotov-Ribbentropp are long gone), so people knew something was going to happen.

Then negotiators tried hard to keep any interested NGO and independent parties from getting access; inevitably, their interest was piqued even more, and leaks started to appear, as they always do when Evil Stuff is in the works.

I'm pretty sure people like Cory Doctorow were banging the drums about ACTA years ago.