Having recharged it and left in on standby since posting first, it hasn't lost much juice at all. I think the guy must have used it...
Mine is very slow, not quite bricked yet, and I had sold it, but he complained about a couple of minor scratches and sent it back. Now I am reconsidering keeping it though, because the replacement I bought for it (Samsung Tab S 8.4) is only a slight improvement really, for the way I use it anyway, which is pretty basic. (Better display, doesn't freeze, plays videos, etc. - but not great battery life and still slow on Firefox and not great with Facebook.) Now wondering about selling my new one...