Mirisa wanders around all night in the Imperial Bridge Inn. Most NPCs in Oblivion sleep and there are beds available in the Inn. I googled and at least according to the UESP the issue seems to be related to the following
Mirisa's packages when at the Imperial Bridge Inn suggest she was meant to be hunting: they are named MirisaHunt1 and MirisaHunt2. However, the packages are of the "Find" variety and targeted at venison rather than deer. As there is never any venison simply lying around, Mirisa will just walk around for twelve hours.
Mirisa was meant to sleep at the Imperial Bridge Inn. However, one bed is owned by Lithnilian and the other two by Davela Hlaren. This means she never finds a bed and instead wanders all night.