They have different meanings to me. If you say someone "should be here by now" (or more likely, "should have been here by now"), I would understand the person is NOT here, and perhaps you are slightly annoyed or worried by the fact that he has not yet arrived. If we are in my huge, four-story mansion and I can't see or hear arrivals at the front door from my dressing room suite, I can say "Oh, he should be here now.
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