The one problem with that is you may develop an infection during that time. Of course you would kill it off when you boiled but it might already have created a funk by then. It is actually one of the shortcuts to try to simulate a sour beer. If you are able to heat it just to pasteurize it before you leave it for a few hours and keep the lid on, then come back and do your boil you should be fine. Just bring it to a momentary boil, and it should take so long to cool that it will pasteurize it.