Something (the person going home) happened sooner than expected. "The spectator went home before the end of the event." Here is a list of the uses of schon that I use for an anki card. 1) in statements, it expresses agreement or confirmation in principle, but with slight reservation. 2) in a response, it corrects what was said and indicates why it was wrong. 3) in statements about the future, it emphasises the speaker's confidence something will happen. 4) it gives persuasive force to a w-question which expects a negative answer. 5) in conditional sentences, it emphasises the condition. 6) in commands, it adds an insistent note. 7) it indicates that something is happening or happened sooner than expected or desired. 8) before a number or quantity, it expresses it is more than expected or desired. 9) when used to qualify nouns, it expresses a restriction.

You may translate "schon" with "even" in this special case: Some spectators left EVEN before the end of the event.

In English, you needn't say "already" (="schon") in this case, but in German it sounds better if you use it.

Some spectators left ALREADY before the end of the event.