I don't know about corporate policy, but the one here locally in my town in MO is not posted as a victim disarmament zone. Now that may not translate for OC as OC is verboten by local ordinance here, but I certainly CC into Kmart every time I go and have even gone in printing pretty badly and have never had anyone comment.
There are several stories in a quick Google search of individual Kmart stores posting following NDs in their stores (thank you irresponsible gun owners for ruining it for those of us who have and know how to use a holster), but I didn't note anything on corporate policy although I could have overlooked it in my quick look. Given the boycott and loss of sales they suffered when they joined the anti-gun crowd a few years ago and quit selling firearms and ammo I doubt they have a no-gun corporate policy. Looks like it is likely a store by store matter.