FWIW, my US regionalism is that “in the hospital” means an admitted overnight-stay patient. Anything else is “at the hospital”. There is a smidgen of ambiguity for the case of somebody awaiting Emergency Department treatment where it’s currently unclear whether they’ll be done & sent home in an hour, sometime well after midnight tonight, or end up admitted before the ordeal is over.
This was @Ike_Witt’s thread so we can see his usage. And yes, he’s Canadian.