ajqtrz
Chef - loquacious Old Dog
Unfortunately, "in hospital" is "in the hospital" because the use of "hospital" as a state of being, is rapidly fading from English. "Hospital" as a state of being is not in general use, probably because we think of hospital as a place a lot more than as as state. At least in American English. And, interestingly, the article is generally not used in the same manner of denoting a location, in oriental languages, so when a speaker of those languages thinks of the location hospital, they hear it without the article. (Other language groups may do the same, I just know enough about oriental ones to say this). So, in fact, they are carrying their grammatical sensibilities into English. All perfectly normal in the great dynamic of language.(More grammar than anyone probably wanted to know this morning)
That's a more complex grammar issue than it seems at first glance, and none of them is wrong, depending what the person intended to convey. "In the hospital" is a location. As in, "he is in the hospital, she is in her office" (or "in the office")
"In hospital" properly refers to the state of being. As in "he is in hospital, she is in office" (state of being). for variation, she is "out of office" (state of being) or she is "out of her/the office" (location)
Many people misuse state of being vs location (mostly by accident, sometimes becuase it conveys more information with fewer words^1) but the phrase itself is not necessarily incorrect without knowing what they intend to say (Alternate example: "He is sick in hospital, she is sick at home"
^1 Saying "he is in the hospital" tells you where he is, but says nothing about why. Saying "He is in hospital" provides the information that he needs to be there. He is not just there as location, but becasue that is his state of being. He is probably sick/injured and has to be in the hospital as opposed to just visting. So "he is in hospital" is a shortcut (weak, but embraced in common speech) for "he is in the hospital because he needs to be there." While Katwick's example conveys location and state in one longer but more accurate phrase: location=hospital, state=in bed.
AJ