Without getting into the merits of whether or not you should have seen the wall or not it appears to me that there is another issue at stake here.
You bumped the car in a car park, you then returned to work, confessed all and went on holiday for a week. In your absence more damage was found to the car.
My question is this…can your employer prove that that extra damage is a result of the bump in the car park? Perhaps someone has driven the car in the meantime? (If you write down the ‘closing milage’ of the car when you leave it it’s a very useful tool for demonstrating the car has been moved in your absence). Similarly can you prove that the extra damage was pre-existing? Did you have a ‘walk around’ the car at the point you took it over?
Given that it’s unlikely either party will be able to prove or disprove any of the above my inclination would be to offer to pay for the damage to the bumper as that was what you agreed to at the time of the incident.
Just some thoughts, hope they’re helpful and as above…it does sound like there’s something strange going on with your boss. Why would he have to ‘have a think about what to do’?