So, this is from an archive in 2001, but this is how I remember hiding in T2A as well.Hiding:
If you're touching or inside of a house, you have a 90% success rate of hiding - even if you have 0 hiding. This is very effective if you're red/gray, need to go AFK or whatever.
Can anyone find any patch notes from 2000-2001 saying Hiding was change? Otherwise, I sort of feel like this is a pretty reliable source.
On UOSA, it *feels* like about 60/40 to hide when next to a house.