No, you didn't miss it. Loopers, their lifestyles, their specific technology (the blunderbuss) and how they make a living seem to be an urban legend with enough substantiated fact, people simply accept it as a matter of course.
Since the Looper phenomemon appears to be limited to a particular part of the city, no one seems to care about its long-term ramifications as long as they get paid to look the other way.
When you have as much money moving though the economy as Loopers appeared to, it would be almost impossible NOT to notice them. But since they had a lot of money, paying ATTENTION to them might be bad for your health.
It was said in the movie that she abandoned her kid at her sister's house and went back to the city to live her life. And since it was apparent in the beginning of the movie that the Loopers loved parties and drugs, it is very possible that some intoxicated Looper at some party told her everything about the industry. Especially since she even knew about closing a Loop.
I'm going to throw everyone for a "loop" no pun intended. If anyone paid attention, Young Joe slept with a hooker whom had a daughter named: SARAH. "Sarah" was the girl looking after Cid. Considering the hooker knew what Young Joe did, and that Old Joe went after the hookers daughter(Note long hair of the kid the hooker carrys to the room, it's blonde.) as one of the three targets this leads you to one conclusion. The hookers daughter is the Sarah watching the young Rainmaker. She herself looped back to change him from a young age, remember Cid stated that, "Sarah's a liar, she's not my mother. My mother was killed." and Sarah stated she was trying to raise him properly. Boom, now you know how Sarah know's what Looper's are.
I caught it the first round. If I hadn't heard the hooker state her daughter's name was Sarah, I would of never of caught it.
Yes - I suppose that 'Loopers" were known to exist due to their conspicuity - but I found this detail a little out of place as well.
On a different point -- I was half-hoping that Cid was a still younger version of our protagonist who had been moved 'up line' to be protected and that the mysterious Rainmaker was in front of us all the time although this would take another bit of plot ratcheting to pull off. I even toyed for a moment with the possibility that the writers had pulled a Rob Heinlein on us and we would going to find out the protagonist had sex with his mother and procreated himself! (perhaps she was grabbed somehow by future Cid and moved around in time to make these arrangements). But alas, this was not the case. Would have made for a bizarro and uncomfortable moment.
...I still want to know how Neo took out those machines in the "real world".
And it looks like Suzie works as a dancer at the club that all them go to (Gat men, looper) so she might have seen something and told her daughter Sarah about the loopers which is why Sarah knows about the loopers