Gun with tapped trigger is not how the device was designed to be stored in holster or operated nor is taping trigger closed an industry best practice.
So, store owner would be responsible regardless if taping was caused by prior customer or attendant (employee) for that matter.
Analogy is ice. Owner has responsibility to ensure ice is not slip and fall risk for customers and employees.
Analogy Fails. Ice is foreseeable and normal condition not caused by negligent / reckless act of third party. Even slippery conditions may not render wash owner liable due to exceptions which vary vis a vis, Open and obvious risk, Natural accumulation, Assumption of risk etc.
I have had a person not put car in neutral and bump another car in tunnel. Sometimes when they exit offender drives off. Bumped car tries to get me to fix their damage. I give them license # of offending vehicle if it can be seen on video. (In one case cops went a got offender). Sometimes driver that got bumped believes I am responsible because it happened on my property. I asked them if they thought the grocery store was responsible if someone bumped them in the parking lot. (I now ask if the city is responsible if someone bumps them on the street) One person said "Yes" I told them to go to whoever gave them their legal
education and ask for their money back because they were taught wrong.
Once had a lawyer send me a letter (amazing how little many lawyers know of how a car wash conveyor operates) with all sorts of BS theories as to why I was liable. I invited him to file suit and I would request that fees be awarded to me for bad faith filing. (I also guessed he must have had some relationship with the customer because his office was quite some distance - different county - from where the wash and customer were located.