I chose hide in the kennels or eventually would say flee and run... but, lets see this logically. Lets say a slave's owner is being attacked (wether the entire city is being raided or just a small attack one on one doesn't matter) and hence he is in danger. What would the slave do? two main options: help his/her owner, or stay away from the fight (choose anything from just curling in the corner to run as fast as hell).
If the slave chooses to join the fight helping his/her owner, if the owner wins, well the slave might be rewarded, or punished for daring to touch a weapon or attack another free... up to the owner. If the owner looses, best case scenario is the slave is punished by the winner in a cruel way, or gets killed.
If on the other hand, the slave remains out of the fight, if the owner wins, the slave collar is unchanged. If the attacker wins, well to the victor goes the spoils and there is just a change of collar. So, considering the risks, and the odds, what would a slave do? Simple, if it is smart enough, to stay out of the fight and let the free solve the issue. And then go and serve paga to the survivors.