You're right, of course.
Though if I started out with that theory, I might be tempted to start justifying it, and think "Maybe it was going to take the intruder too long to get in and out, so s/he brought a second Polyjuice potion to take in the middle... and maybe s/he left the potion figuring it would give away the game if 'Harry' were caught with a Harry Polyjuice potion on 'him'... and maybe s/he left it in Bellatrix's cell because, um, because s/he couldn't find a better place to hide it... in fact, maybe s/he really thought the cloth would hide it... and didn't crush it because, um, s/he's incredibly stupid... no, um, wait, because... um... give me a minute, I'll come up with something."
So, yeah, OK. The only way I would actually consider finding such a flask convincing would be if I were only thinking about the problem very superficially. Perhaps Quirrell is just counting on the investigators not being very careful thinkers. Lord knows the wizarding world is full of those.
Alternatively... if the potion were a Bellatrix Polyjuice potion, I might conclude that this is someone else's corpse, and Bellatrix was never arrested in the first place, and whoever it was died shortly after being arrested, and expected the vial not to be noticed.
But why that conclusion is worth leading someone to, I don't know. And why not just leave a different body to begin with? So, probably not.
So, OK, turn it around. What potion, if found by the Aurors near Bellatrix's corpse, would lead them to a useful-to-Quirrell conclusion?
(shrug)
Of course, another possibility is that the flask wasn't meant to be found... maybe it was actually well hidden, and Dumbledore is just awesome, and Q didn't expect D to be involved. That would suggest the flask is performing some actual functional role... beats me what it could possibly be, though. Maybe it's meant to explode upon being investigated, destroying "Bella's body."
Perhaps Quirrell is just counting on the investigators not being very careful thinkers. Lord knows the wizarding world is full of those.
But why leave the flask at all, then? It must be meant to make them think something else.
whoever it was died shortly after being arrested, and expected the vial not to be noticed.
Couldn't be. Bella's been there for twelve years - and they've been feeding her daily, they'd have noticed if she died "shortly after". And the fake body doesn't look ten years dead.
...What potion, if found by the Aurors near Bella
- This thread has run its course. You will find newer threads in the discussion section.
Another discussion thread - the fourth - has reached the (arbitrary?) 500 comments threshold, so it's time for a new thread for Eliezer Yudkowsky's widely-praised Harry Potter fanfic.
Most of the paratext and fan-made resources are listed on Mr. LessWrong's author page. There is also AdeleneDawner's collection of most of the previously-published Author's Notes.
Older threads: one, two, three, four. By tag.
Newer threads are in the Discussion section, starting from Part 6.
Spoiler policy as suggested by Unnamed and approved by Eliezer, me, and at least three other upmodders:
It would also be quite sensible and welcome to continue the practice of declaring at the top of your post which chapters you are about to discuss, especially for newly-published ones, so that people who haven't yet seen them can stop reading in time.