I don't like to blame Minerva. Even if she was thoroughly incapacitated at the time, we don't know that she missed the opportunity to do anything. Unless she'd happened upon him sweeping up and down the corridor outside the dungeons, she'd likely never have known he was there, if he's found a way to circumvent the wards.

I know you never liked him, but you can't deny, if a way into that castle exists, he's more than clever enough to have found it no matter how strong the wards are.

Just make sure the children don't get to pass on the legend of The Time The Evil Sirius Black Was Caught On School Property And Promptly Kissed By A Dementor.