OK. So:
1) Start with adding the unit to a "hidden in bushes" unit group when they enter a "bushes" region.
2) Turn on a periodic trigger whenever the unit group has greater than 0 units in it.
3) Check all units in the unit group every 0.25 seconds or so (periodic event). If the unit is attacking or moving, remove the Permanent Invisibility ability. Else, add the Permanent Invisibility ability. Set the fade time for the ability and unit to something appropriate for you.
4) If the unit dies or leaves said region, remove it from the unit group. Turn off the periodic trigger if number of units in the unit group = 0
You'll need a total of 3 triggers for this.
I'm actually doing something similar for terrain effects on one of my projects, where forests hide certain units