You'll find various analog ICs in SOT23 package, e.g. voltage references or reset generators, so that's definitely an option.
Marking is manufacturer specific (except for some popular parts where many manufacturers are using the same marking, e.g. said A7s = BAV99).
Identifying circuit function is an interactive process, so I fear we can't support it effectively from a distance.
Knowing the instruments purpose, origin and age can possibly narrow down the manufacturer and type choice a lot.