We are planning to take a trip to the USA using Korean Air and were thinking of stopover in Seoul for 2 nights. We will have 6 pieces of luggage so we want to store 3 of them at a luggage storage airport facility. I was wondering if these facilities are secure and do they normal guarantee against theft. I checked the website of one of them but it was in Korean, surprise. Since this is Korea can I "assume" that luggage theft might be low?

I would "assume" it would be secure.  But you never know.  I'd take anything of high value with me.

 

http://www.sleepinginairports.net/asia/seoul.htm

 

http://www.airport.kr/airport/facility/efalicityInfo.iia?carId=23&facilityId=84

Inchon is a modern and very secure airport.  I left my bag at Baggage Storage while I did a day tour of Seoul  between flights and nothing was tampered with.

 

BTW, Inchon airport is nice in itself.  the departures area has free showers, sofa beds (in the upper mezzanine "Rest Area") a small but cool cultural museum and several handcraft kiosks where they show you how to make some of your own creations free of charge.  Also live music and dance performances.

One of the few airports I know (Changi in Singapore is another) where a 3 hour layover is too short.