Ah, I found it..
http://www.myblueant.com/downloads/m...erGuide_US.pdf
Page 13: "If you take your phone out of range of the S4 for more than 2 minutes, the handsfree trigger switches off and “sleep mode” is activated.
When you come back, you must tap the BlueAnt symbol to reconnect your phone and reactivate the trigger"