Autoretreat widget handles this, not patrol.
Patrol merely sets a fight command from one point to another, automatically toggling tactical AI. Autoretreat does what its name says: it automatically retreats damaged units. By default all units will fight to the death, however, there is a widget to set all units to retreat at 66% health or something like that. Once the unit starts retreating, it will stay in a designated repair area or land on a repair pad until it is fully healed, after which it will resume its last command.
Its the little chicken icon on the side of the command card. Once the unit reaches 33% / 66% / 99% or something like that, it will automatically fall back to a repair pad or nearest designated repair area (there's a command to designate one in the command card.. forget the icon though).
Hope this helps clear things up.
