On a recent review audit on SU, I clicked "Edit" and was greeted with the plain text "This post was deleted and cannot be edited". It wasn't in a popup box or anything; the actual page consisted of just that text, with no formatting (as though it were the only content of the HTML page). This makes it really obvious I'm dealing with an audit.
As mentioned over on this other question, this has been fixed. If you click "edit" you should now get a notice saying you passed the audit.
This should probably not be considered a bug.
If someone clicks Edit on a post, it's difficult to know their full intentions, but probably they think something is wrong with it that is worth fixing. Most of the time, this could be a reasonable action to take. Once someone clicks edit, they'd usually pass the audit.
Suppose someone clicks the link link to view the post (outside review). They'll see it's deleted then, too. Clicking that is a more efficient way to check if a post exists than trying to edit it (see point 4 below for details about that). Nothing should be done about that (non-)issue, and for the same reasons, nothing should be done about the user discovering a post doesn't exist when they click to Edit it.
Even if people's edits are bad (perhaps especially then), we don't want people spending lots of time writing edits that can't be submitted. This is why editors are notified when a post is deleted while they're editing. Having people spend time editing a nonexistent post is cruel and pointless.
The current behavior is actually quite reasonable. Clicking Edit usually lets you edit the post. When the post is deleted and present as an audit, clicking Edit gives you a non-user-friendly page and requires you to use your browser's back button, or similar facility, to return to reviewing. So this is a very inefficient way to determine if a post is an audit. Imagine someone clicking Edit on everything! That would likely be slower than actually paying attention.
Arguably, this is a low-impact interface bug. It is not a hole in the review system. If this is considered a bug and fixed, the fix should still inform the user that the post cannot be edited.
Potentially, instead of saying it's deleted, it could say that someone else is editing it (since under some circumstances that prevents a new edit from being started). People would get wise to that almost immediately, though, and even if they didn't, they're not very likely to fail the audit if they thought the post needed editing and were unable to edit it.
Therefore, there are two good responses to this.
The best response would be to fix the minor interface bug by making Edit tell the user that the post does not exist, but in a way consistent with the Stack Exchange interface.
This should probably be a little bit slow. That way, no incentive (compared to the current situation) is created for people to click Edit to determine if the post is an audit.
One option would be to actually bring up an edit screen, but with a bar at the top informing the user that the post has been deleted.
But I think a better way would just be to introduce 2 or 3 second delay after clicking Edit (which sometimes happens anyway) and then tell give the user one of those little message windows (which are blue, currently) to tell the user it cannot be edited because it has been deleted.
Another completely acceptable response, as argued above, is to leave the system's current behavior as it is.