Must be something significant.
He had been traveling by trams since he was in his mother's womb and yet he had never recognized there was such a tablet, a birthmark of some history. 1904. What happened in this year? Must be something significant.
I know there are some other problems in this paragraph, but my focus is on the red sentence. Obviously the writer is referring to something past; I think he should have said 'must have been something significant'. But I hope to know whether it is ususal or grammatical to simply use MUST BE in this context.
I am studying at university in Hong Kong and major in English.