9 times out of 10 when we see posts like yours the expired offers are Seller Initiated Offers. In other words, they are offers you sent to buyers that had the item on their watch list or in their shopping cart and those buyers did not accept the offers.
If a buyer initiated the offer you would be able to see their ID on the offer.
If a buyer initiated the offer you would see it in several places from eBay messages to eBay notifications to the pending offers section on the Seller Hub Overview page to e-mail alerts if you have them set up.