https://github.com/MOUZU/Blizzard-WoW-Interface/blob/d162a4c0d198a4381b5b6573d975635ed7316702/1.12.1/FrameXML/ContainerFrame.lua#L52
Not tested, but try:
local itemIdSearch = 3467;
local bag = KEYRING_CONTAINER;
for slot = 1,MAX_CONTAINER_ITEMS do
local itemLink = GetContainerItemLink(bag,slot);
local _, itemCount = GetContainerItemInfo(bag,slot);
if itemLink and itemCount then
local _,_,itemId = string.find(itemLink, '.*|Hitem:(%d+):.*');
if itemId and tonumber(itemId) == itemIdSearch then
print("Key found");
return
end
end
end
print("Key no found");