So this is the first major bug I've come across in my single-player game... unless I am missing something really stupid here.
I've just escaped from Spellhold, it was night-time when I did so, so I went straight to Saemon in the tavern, and from there to steal the pirate horn. I expected to find Saemon waiting at the docks when I returned, but he wasn't there.
I rested, and waited until the following night, and Saemon is still not there.
If it matters, I am playing on PC, Beamdog, Windows 7. Only a few cosmetic components from BG2 Tweak and SCS in terms of mods.