Causes for random facebook cookies deleted?

My dad went out to get some coffee for about 15 mins. When he got back he went on facebook and his facebook told him that his computer wasn't named for that account. He says this is because cookies have been deleted and is accusing me of deleting them on his computer. I didn't use his computer! I try to... show more My dad went out to get some coffee for about 15 mins. When he got back he went on facebook and his facebook told him that his computer wasn't named for that account. He says this is because cookies have been deleted and is accusing me of deleting them on his computer. I didn't use his computer! I try to tell him there must be a mistake but he doesn't care. He says he KNOWS what must have happened, even though I KNOW I didn't touch his computer at all. He now thinks I'm a liar. I need help! Any reasons for what happened?
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