This exact thing happened to me last summer where my cf front lip was smashed up by bmw dealership. After they brought the car to me I started to drive away then occurred to me to do walk around b/c someone had just posted about some damage on forum few days prior. Shocked to see the damage and went right back to service desk where thankfully they ended up taking care of everything (ordering me new lip and getting it installed for me).
I know they reviewed shop camera footage before replacing the lip and so as suggested by others would ask that they do this for you. Unfortunately short of this, once you drive off lot, especially day later, you cannot prove damage didn't happen somewhere else or hold them accountable.
At this point, would do exactly what sroc5 outlined above. Sorry to hear though!