I had ordered a bit of acid for my friend and I for a trip in the beautiful weather today. My friend came over at the same time as the postman was putting mail in the boxes (I'm not that worried about sending a tiny amount of acid to my home) and he asked if there was any mail for me to which he said no. He had left a letter in our mailbox though, but it was addressed to the girl next door. The postman admitted fault and put it in the right mailbox, inches above mine.
We figured that since the acid wasn't delivered to us he might have left it in her mailbox as it would match up delivery time that it came today and we really want it today. So we opened the slot and peeked inside her mailbox and sure enough a suspiciously unsuspicious looking letter was lying under the letter that had been in our mailbox. We're not sure but it's not completely unlikely that it might be ours.
Just try to catch her outside and ask if she'll show you her box. Tell her something of yours may have accidentally slipped into her box.