The bottom LED near the ADAT blinks to indicate FireWire activity. If is is steady, this is because of .. no activity. In your case you should ignore it.
The EXT light flashes if any routed digital input is unlocked. Note I said routed, not used. Because there are very many digital inputs, it is quite likely that one or more of these is routed even if it is not being used. In your case, if you are using no digital input, you can ignore it.
If the flashing annoys you, you will have to go through all the routing pages and turn all the digital I/O off. This will be character-building and educational.