Have a look at the Incheon website
From the airport website in the FAQs (took some finding):
(
http://www.airport.or.kr/helpdesk/)
"With regards to your inquiry, there are baggage store service near west or east side on the 3rd floor.
You should pay for using baggage store service, it costs about $8(USD) per 24 hours, but it depends on your baggage's size and weight.
They are open at 6 a.m. to 10. p.m. and everyday is available
For further inquiry, please contact baggage store service(82-32-743-5804) or the information desk.