This is such an awful story, and why I never valet my car. The problem is that you didn't do a once around your car, which I always do after leaving a dealership, car wash, or my work parking lot (which I only park in front of the cameras).
If you approach valet, they will deny it, and will argue you did it as soon as you left.
It looks like they hit hard enough to damage the bumper, not just the cover, so you're probably looking at an insurance claim.
Sorry about your story, it sucks