While invisible, you can’t be seen. You’re undetected to everyone. Creatures can Seek to attempt to detect you; if a creature succeeds at its Perception check against your Stealth DC, you become hidden to that creature until you Sneak to become undetected again. If you become invisible while someone can already see you, you start out hidden to the observer (instead of undetected) until you successfully Sneak. You can’t become observed while invisible except via special abilities or magic.