THe 648 seems to be detecting some silence?

only in ADAT 5 ... the green led fails to turn off when the decibels fall back down. 
Totalmix shows that there is no activity in any channel ... but the adi 648 is picking up something. When i press play music comes out of that ADAT out.. the LED turns on the 648, but then when i press stop the music dies down and the LED doesnt.  Strange anomoly.