Como a resistência foi exposta?

Eles foram expostos porque enviaram um vôo de reconhecimento para encontrar a base de Starkiller ...

“The scan data from Captain Snap Wexley’s reconnaissance flight confirms everything Finn has told us,” Poe announced to the group.
The Force Awakens novelization by Alan Dean Foster, Chapter XIV

... e esse voo foi rastreado até a base deles:

Hux was clearly gratified to be the bearer of good news. “We were able to track their reconnaissance ship back to the Ileenium system. We are coordinating with our own reconnaissance craft in the area in order to lock down the specific location of their base.”
Snoke replied with cold satisfaction. “We do not need it. Prepare the weapon. Destroy their system.”
The Force Awakens novelization by Alan Dean Foster, Chapter XV

e

The general halted before the dais and bowed. “We were able to track their reconnaissance ship back to the Ileenium system. We are coordinating with our own spy vessels to lock down the specific location of their base.”
“We do not need it. Prepare the weapon. Destroy their system,” the Supreme Leader said.
The Force Awakens Junior Novelization by Michael Kogge, Ch 16

A Primeira Ordem descobriu a sua localização antes da destruição da sua Base Starkiller no filme anterior. Não tendo conseguido destruí-lo com sua super arma, recorrem a meios mais convencionais; enviando uma grande frota de navios para explodir a partir de órbita.

GENERAL HUX: We have their location. We tracked their reconnaissance ship to the Ileenium system.

SNOKE: Good. Then we will crush them once and for all. Prepare the weapon.

_{^{Star Wars: The Force Awakens}}

Como um lindo easter egg, finalmente sabemos como o First Order conseguiu rastrear o navio da Resistance de volta a sua base, presumivelmente usando o tecnologia de rastreamento de hiperespaço que aparece em The Last Jedi.