When we observe an object for a while, then stop observing, and then take up observation again, we don't know what has happened to the object between those two periods of observation, so its state at the start of the second period of observation is not deducible entirely from ts state at the end of the first period of observation. There is an epistemological uncertainty involved.
a quantum object interacts with a piece of apparatus and its state is
recorded by that apparatus, we can determine what the state of the
object was at the time of interaction. If the object is allowed to
continue without interaction with any other object whatsoever for some
finite duration, then there is no record anywhere of what
happened to the object between that interaction and its subsequent
interaction, so its state on subsequent interaction is not deducible
entirely from its state on the prior interaction. However, it seems to
be the case that this is an ontological uncertainty rather than an epistemological uncertainty.
I've come to regard this "superposition of states" for quantum objects as more of a superposition of histories.
In the case of a quantum object passing through a single slit and
meeting a detector, the superposition of histories is constrained to a
single slit, so the detector only sees particle-like behaviour. In the
case of a quantum object encountering double slits and meeting a
detector, the superposition of histories has to take into account both
slits, so the detector sees wave-like behaviour.
scenario is a highly constrained case of the quantum object "going
everywhere" (i.e. a case where most of "everywhere" has been narrowed
down to two slits), this notion of "going everywhere" being what I take
to be metaphor for "having all possible histories". It may even be the
case that a non-interacting quantum system can be considered to be constituted by all of its "possible histories". This interpretation would also be consistent with the "delayed choice" and the "quantum eraser" variations of the two-slit experiment.
Note that human consciousness in the form of a human "observer" is not implicated in these scenarios, but the mystery of ontological uncertainty remains.