If a dog eats a grape then nothing will usually happen to the dog.
A dog usually has to ingest more than a few grapes before the grapes become poisonous to the dog.
Grapes won't kill a dog simply because the dog ate a few grapes and the death will not happen right away either.
The way grapes kill dogs is through the grapes causing kidney failure in the dog which happens overtime and not overnight.
Most times a dog will be just fine eating one or two grapes if they ate it by accident.
Don't feed your dog grapes but if they happen to eat one or two grapes and are not throwing up etc then they are likely just fine.
My own dog has eaten a few grapes every now and then by accident and he's just fine and has no kidney issues.