Where's the keys?

After starting work for a couple of days, I was feeling the lethargy from travelling to work by public transport. The season parking label for my company is not available yet, and parking in the city costs at least $12 a day. So the best option was to travel by public transport.
When I reached my apartment block, as usual, I checked the letter box, then took a lift to the 10th floor. Upon reaching the 10th floor, I found my keys missing! Thinking that I must've left it in the mailbox, I rushed downstairs to get them. When I reached there, the keys were not there. So I rushed back into the lift, went back to the 10th floor and searched almost everywhere.
Finally, I decided to give the ground floor another look. Just then, I remembered throwing away an advert slip that I collected from the mailbox. I looked into the garbage bin beside the lift, and viola, there, in the bin, was my keys!