It would be interesting find abandoned bicycles in the city and to place signs on them saying something like, “Whom have you forgotten” The rusty, beat up, lonely bicycle would be used as a metaphor for those that we have forgotten in our lives. However, I’m afraid that the sign may tie in too directly with the bike. The bicycle would take on a personification role where the viewer merely thinks that the bike was forgotten by someone… begging the question who forgot their bike? Which might be an interesting thought too.
But it would be even more appropriate if the sign was to be put on something where you would normally put a sign onto… Like people normally don’t go around putting signs on abandoned bikes. But there may be signs at a construction lot… A sign that looks kinda like a permit. But actually the sign at the construction site is asking the question, “Who have you forgotten?” Tieing in a connection with the past history of that site and what is forgotten there. but also using that as a metaphor for someone forgotten in their your life, because you have torn something down.
Actually, I kinda like the abandoned idea better. Because it’s something that was just slipped away. It wasn’t necessarily a conscious choice, like construction.

there are lots of abandoned bikes in Lakeview.
Is it better say… Whom have you forgotten or Who have you forgotten The “whom” is proper grammar, but sounds rather stuffy. Also, do you think there should be a question mark at the end?
i like the “whom” – it sounds formalized. yes on the question mark.