We too have this problem. We have a female (desexed, approx 3 - 4 yr old) mastiff that when she gets out of the yard, whether it be out the gate (accidentally left open) or over the back fence (property is under development in the next few years), she takes the bolt - but only when we realise she is out. Once she wandered around the front yard (not fenced) from when our daughter left at 7.30 until i noticed her through the fence at 9 o'clock. As soon as we call her she's off down the street, never the same one, just in the direction she is facing at the time of discovery. She will run for a little while and waits, smelling or going to the toilet, until we are about 2 metres away then off she goes again, once i chased her for over an hour and it took all of my patience to not go balisitic at her when i caught her. I do realise she thinks this is just a game but i am absolutely petrified that she may be hit by a car or go for another dog that is roaming (she is quite social, but still a fear of mine), so i won't risk it and just turn away to bluff her. She is quite well behaved in the yard and does come when she's called & will sit but i have not really instilled in her these 'rules'. She is booked in for dog obedience but this does not start until the end of July. I will try some of your suggestion but any more would be greatly appreciated.