1. No, they did not know about Starkiller base before.
TFA novelization by Alan Dean Foster establishes that (and, ironically, answers your presumably sarcastic quip about "Surely a patrol or a trade ship would have noticed something" ! :)
In the main conference room of the base on D’Qar, an ongoing strategy session had brought together the leaders of the Resistance. Leia, Poe, C-3PO, Han, and an assortment of senior officers including Statura and Ackbar were assembled around a three-dimensional map of an isolated, frozen planet that up until now had not been worth a hopeful visit from a minor trading ship. Finn was present, too, since it was his information about the world in question that had prompted the gathering.
“The scan data from Captain Snap Wexley’s reconnaissance flight confirms everything Finn has told us,” Poe announced to the group.
Wexley spoke up. “They’ve built a new kind of hyperspace weapon within the planet itself. Something that can fire across interstellar distances in the equivalent of real time.” His expression showed his incredulity. “I’ve had my share of technical training, but I can’t even imagine how that’s possible.”
2. If you look at the Galaxy Map in the Visual Dictionary, Starkiller base isn't exactly where everyone would be likely to look.
Starkiller base is not only out in the middle of nowhere on the edge of the Galaxy
It's in the Unknown Regions specifically, where few people go.
There was a reason First Order retreated there.
It's directly on the opposite side of the Galaxy from the Rebellion's center on D'Quar
3. How would anyone know what they are looking at anyway?
It's NOT a giant metal moon. It's a real planet.
You wouldn't know something was amiss even if there was a trading ship around (and as shown in #1, there wasn't).