I would have done that except I didn't realise until way way way too late. Lydia, unlike the other companions I tried to replace her with, was actually useful and would sneak and not many any noise and wouldn't bump me into an enemy I'm trying to stealth kill. So I just assumed she was doing her job until one day she wasn't there any more. I went back to the last place I remember seeing her - fighting the dragon at Kynesgrove - and there was her beautiful self, lying dead on the ground. I wept.

Also Shadowmare or whatever the Dark Brotherhood's horse is called: it died somewhere and I can't find it to make it respawn. Gosh.