Oh, you're quite right. Perhaps Quirrel was planning to kill the Auror to make it clear that a break-out had occurred? That way, a full check of the prison would occur and Bella's replacement would likely be found. Which in turn would mean that it was put there simply to deceive Harry into a false sense of security. When the break in is made public, Dumbledore would naturally come under suspicion (since a Dementor disappeared under his watch) and he would suspect Harry. That might also explain the lack of the 30th charm by Quirrel. Might make Harry traceable.

I could be completely wrong, of course. Pure speculation.