How do you explain a failed inspection to a client when the issue is minor?

I offer my clients a 12 month warranty on my pest control service (rodents) if they agree to have me complete all the exclusion work I recommend. Then I kill off what ever population is in the house and walk away...95% of the time.

If they see activity after 2 weeks that is pretty much me saying I missed something and failed the inspection. It's not my favourite way but its my only way. People are never angry or upset because the current population is going to be lessened, and everyone knows what a persistent mouse can do. I will show up again and make sure I do a good second inspection and ensure nothing was missed. A second pass of the house in the opposite direction or from a different angle does sometimes help anyway.