What can I do about the invisible notices on the Bulletin Board?

Start by visiting the Solicitors, as the only visible notice suggests.

Have you found a KEY and a NOTE there? If not, return when you have.

If you have, you'll know to head to Gio's Apothecary.

Use the key on the locked cabinet.

Grab the IODINE bottle from the cabinet.

Use the iodine on the Bulletin Board.