Also I am willing to give away one of the lacations that I checked today: On a hunch I went to the bottom of Drachenfall Waterfall, I wouldn't put it past SE to stick one in there, as no one ever goes down there anymore ;0)
I got one in Valley of Sorrows at about 10am PDT. Is there some delay between the time that they are lost and when they spawn again? According to the chat log of a mule I had in Rolenberry Fields the tablets were lost ~7 hours earlier so I thought I'd look and just got lucky. The first one found on Hades was just a few min before I found mine too.