what have they seen

 Then said he, What have they seen in thine house? And Hezekiah answered, All that is in mine house have they seen: there is nothing among my treasures that I have not shewed them.
[5] Then said Isaiah to Hezekiah, Hear the word of the LORD of hosts:
[6] Behold, the days come, that all that is in thine house, and that which thy fathers have laid up in store until this day, shall be carried to Babylon: nothing shall be left, saith the LORD.

 

I've generally thought that the Lord God Almighty, maker of heaven and earth, was rebuking Hezekiah, but that's probably not it. God was going to send everything to Babylon no matter what the king did. God was simply telling the future, and laying a guilt trip on Hezekiah.