You might have left it on the bus and hopefully someone is nice enough to give it to the bus driver. For now, try going to the HSR Lost and Found place.
Lost & found
If you leave something on the bus you can call 905-528-4200 and ask for Lost and Found, or go to the HSR Customer Service Centre, Hamilton GO Centre, 36 Hunter Street East. The office is open Monday to Friday from 8:00 am until 4:00 pm. If you find something on the bus, please give it to the bus driver. Lost and Found items left on buses on any given day are not available for claiming until 11:00 a.m. on the next weekday.
http://www.hamilton.ca/CityServices/.../RideGuide.htm