A little while after I got off the phone with them, my dad called back and said that about 60 years ago, when he was around 12, he and his friends used to play in an old "haunted mansion" on his street. Basically, a big old abandoned house that they were all scared of (he said that they had theories that their grade-school janitor lived there [I remember having similar theories about my grade-school janitors...]) and that they used to vandalize. He said it was about 3 or 4 stories and they used to go up to the top and drop furniture into the basement to watch it explode. He said they once found a big mirror ("about as tall as a person") and they dropped it down and it was the biggest explosion he's ever seen.
He proceeded to tell me that one day he found a big bunch of skeleton keys and he was convinced that they were the keys to a treasure somewhere, and he had held on to them ever since. Yesterday this box arrived in the mail:
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This morning I went down and tried the keys on our bathroom door. The one that I marked with blue tape worked.
Thanks, Dad!
(Your treasure will be waiting for you in our bathroom.)