Sighting at the mall, with the info available is like most other details here - you can reasonably argue against the likelyness of it being accurate as well, as justify why it seems absolutely valid.
No sighting at the Minot then. Rachel's stepson's mother reported driving Minot Avenue sometime after noon and spotting the Oldsmobile on the driveway, not the girls.
And of course we don't know what kind of statement it was. And could be as well:
a) her going somewhere, at unspecific time between late morning and early afternoon, seeing similar car on the driveway or even the Oldsmobile before Rachel left and recalling that only after learning about girls disappearance,
b) her going somewhere, for scheduled meeting at 1:00 pm, knowing exactly that she left home 12:15 cause stared at the clock waiting for someone to come back home, then driving Minot with no distractions, having 100% confidence in recognising that car and taking mental note like "oh, that's odd, TT's car, I never saw any car in the driveway at this time of day before".
I guess that both possibilities would lead to a conclusion that there was a sighting, but the weight between a and b is very different.