In The Final Empire, after Kelsier and Vin go to meet up with Marsh for the second time since he joined the Ministry, they get to the rendezvous to find what they believe to be evidence that
Marsh has been tortured and killed by the Steel Inquisitors.
Once they find this out, they send a message to Lord Renoux to shut down their operation and hide at the safehouse.
However, on the way, Renoux, Spook and the staff
are captured and sent to be executed.
We find out at the end of the book that
Marsh has not been tortured, but has instead been turned into a Steel Inquisitor, and has remained loyal to the crew, which is shown when he kills a number of other inquisitors.
Marsh never gave up the crew,
how is it that Renoux was discovered to be a spy?