Get a fault code reader off eBay around £15 and find what fault it has. When disconnecting the battery you are wiping the memory clean hence no fault until you run he car again which will then pick up the fault and log it in the memory then turn the light on again.
When the car first starts the imrc lever opens for a few seconds then returns to the closed position until the throttle reaches 5/5500 rev which is when it changes the inlet runner length.
Have you tired the Focus diagnostic test

If not keep your finger pushed on the mileometer reset stalk for a few seconds and start your engine both together , this will do a self diagnostic test , then press the stalk until you get to the dtc part if there are any faults it may have picked them up. The problem is sometimes it doesn't pick the fault up but it's worth a try.
If you get a code have a look on web by typing Focus st170 fault codes and you should be able to find out what up with it

Hope this helps.

Cheers paul