To alleviate paranoia run this script every minute through your crontab and you will get an alert if unknown IPs are connecting to your Bittorent Sync folders.

unknownBTConnections.sh

#!/bin/sh
######################################################################################
# The location of your Bittorent Sync log
LOG_LOCATION="/home/user/.btsync/btsync/sync.log"
# The list of known IPs you do not require an alert about
KNOWN_IPS="192.168.1|69.147.76.15|216.239.51.9"
# The icon for your alert
# I found a great little image here:
# http://i1-win.softpedia-static.com/screenshots/icon-60/BitTorrent-Sync.png
ALERT_IMAGE="/home/user/bin/BitTorrent-Sync.png"
######################################################################################

cat $LOG_LOCATION  | grep Incoming | cut -d ' ' -f6 | cut -d ':' -f1 | grep -Ev $KNOWN_IPS | sort -u | while read ALERT_OUT; do DISPLAY=:0.0 XAUTHORITY=~/.Xauthority notify-send -i $ALERT_IMAGE -t 5000 --hint=int:transient:1 -- 'Unknown BTsync Connection' "$ALERT_OUT";done

If after a while you start getting alerts about an unknown connection but the pop-up shows no other information you will need to delete your "sync.log" file and restart BitTorrent Sync.