In what contexts might "風呂" be used instead of "お風呂"? "お巡りさん" has been lexicalized. "お茶" is almost lexicalized. What about "お風呂"?
Off the top of my head....
In scholarly and/or technical writing regarding bathing or baths.
In advertisement for apartments, describing whether they are equipped with bath tubs or not.
In the news about a bathroom.
Regardless of the context, many male speakers choose to use 「風呂」 over 「お風呂」 on a daily basis.