How does ground realize transponder/ACARS turned off?
I'm writing a TV show about a woman who investigates plane crashes for the NTSB.
In the pilot episode (pun intended), a plane goes missing. Everyone assumes it crashed from mechanical failure ... until they realize the transponder was turned off and they start to suspect foul play.
My question is -- how would people on the ground know someone turned off the transponder (and that it didn't just shut down from a catastrophic failure/crash)?
Also, how long would it take for people on the ground to piece this information together?
Many thanks!