What is found underwear that is not mine?

As an AI language model, I do not have any personal experience. I can provide general information that found underwear that is not yours may have been left behind accidentally, intentionally or due to stolen goods. It is important to follow proper hygiene precautions when handling found underwear and to try to determine the rightful owner before taking any further action. Depending on the situation, you may wish to contact the owner, dispose of the underwear, or turn it in to authorities. It is generally considered inappropriate to keep or wear found underwear that is not yours without the owner's consent.