How do we know that nobody has ever seen a person in the tower in "The Lady of Shalott"?
The following lines reveal that nobody has seen the Lady of Shalott alive when she was busily "weaving by night and day/A magic web with colours gay."
"But who hath seen her wave her hand?/Or at the casement seen her stand?" The answers to these two rhetorical questions is, "no one." This is because, she is under a spell and has to constantly "night and day" weave the "magic web."
On seeing the reflection of Sir Lancelot in her mirror, she stops weaving and gets up from the loom and crosses her room and "look'd down to Camelot." Immediately the curse comes upon her and she realises that she will die soon. She lays herself in a boat after writing on its prow 'The Lady of Shalott' and sails down to Camelot. By the time she reaches Camelot she is already dead. Everyone in Camelot comes out to look at her dead body but they are not able to recognize her and are puzzled by the legend inscribed on the prow of the boat: "Knight and burgher, lord and dame/And round the prow they read her name/The Lady of Shalott/Who is this? and what is here?"
The fact that no one in Camelot is able to recognise or identify her dead body is clear proof that nobody has seen anyone in the tower.