If your vehicle was struck by another vehicle in the parking lot of Walmart and that person did not stick around or write you a note as they legally should have then this would be considered a hit and run.
Since it would appear that someone backed into your car with their car and hit your trunk area this would normally fall under your collision coverage and not your comprehensive portion of your physical damage coverages.
Comprehensive covers your vehicle, and sometimes other vehicles you may be driving for losses resulting from incidents other than collision. For example, comprehensive insurance covers damage to your car if it is stolen; or damaged by flood, fire, or animals. Collision covers damage to your car when your car hits, or is hit by, another vehicle, or other object so this is the coverage to use in the case of your parking lot accident with the unknown driver.
Collision coverage pays to fix your vehicle less the deductible you choose. So even though you were not at fault you will still owe the deductible associated with this coverage.
Uninsured motorist (UM) in Florida is for bodily injury not property damage. UMPD is available in some states and would cover damage to property for hit and run damages. In FL UM (Uninsured Motorist Bodily Injury) covers bodily injuries that one sustains if hit by a person that was driving without insurance so it would not cover the damage to your vehicle.
Florida is a no-fault state however this does not cover physical damages to a vehicle but bodily injuries. No-fault in Florida really is referring to PIP coverages and your rights to sue for personal injury, personal injury protection, insurance which drivers are required to carry. This means if you are injured in an accident your own PIP policy will be used for your bodily injuries.
Fault can be found with regards to damages to your car. If the other person who had hit your car had left a note or stayed around for you to get their information then you could have placed a claim with their insurance company since that person would be found at fault for backing into your vehicle. Since they did not then you will need to claim this with your own insurance company under your collision coverage.