> most devices like this use motion sensors to go into low-power mode when stationary and only transmit on the move
I've been working with (non-covert!) tracker devices for a project, and use exactly this approach, when stationary the tracker goes into low-power mode and sends position once every 12 hours to preserve battery life. When motion is detected, we send regular updates.
If low cost is the goal, consider a voltage measurement device. ICE engines have electrical systems that run ear 13V when the engine is on, and ~12.5-12.8V when the engine is off
I've been working with (non-covert!) tracker devices for a project, and use exactly this approach, when stationary the tracker goes into low-power mode and sends position once every 12 hours to preserve battery life. When motion is detected, we send regular updates.