And now you're going in circles again; tweaking plot instead of designing a character to fit the plot. At this point, my recommendation to you would be to start over because all of these issues should have been, and would be better, addressed during development; specifically, during the outline phase at the latest, but even earlier would be better. In any case, the script stage is much too late, which is why you're having the issues you're having now.
You didn't Google "MacGuffin", did you? Well, just to recap/clarify before I bow out of the thread, you expressed concern that the cop might not take the witness back to her place to pack a bag. So I asked "Why are you restricting their motivation for returning to her place to packing a bag?", as opposed to something else that would give them a stronger motivation to return to her place (i.e. a MacGuffin) like retrieving the stolen Death Star plans that are on her laptop, or the time-traveling DeLorean parked in her garage, or the last Infinity Stone that she hid in her jewelry box, or whatever else. I said nothing about placing the MacGuffin in a bag. (Although, putting a DeLorean in a bag won't make it any easier to carry around.)
And one last sidenote:
This also supports the "coincidence [that] makes it more difficult for your protag to get what s/he wants, it enhances the drama" thesis because, when someone gets into an elevator, their (immediate, but minor) want/goal is to get to another floor in the building. Getting stuck between floors works against that.
Oh sorry, I don't mean to go in circles. This wasn't a problem before when I wrote the script, and I wrote it a certain way. It's just later after putting it away for a few weeks and looking at it with fresh eyes, something about this section of the plot seems dis-satisfactory or clunky. I can't put my finger on what it is, it's just something feels like it could be better about it. That is why I asked which scenario is better. But maybe it's all in my head and I should stick to the original writing?
And yes there is an important item she wants to pack in her bag, it's just she cannot tell the cop about that item, so she has to convince the cop to take her back there, to pack a bag, without telling him what the item is that she wants to bring along.