Swings do not always cover their gender. It depends on the needs of the show and whether a certain character needs to be defined by their gender. ie. Dynamite in Hairspray is sometimes covered by a guy
Also, when a swing goes on, it does not have to be noted with a slip in the playbill or a sign in the lobby - since they are replacing an ensemble member rather than a specific character.
I've seen Les Miz enough times to know when a swing was going on, regardless of whether it was announced -- but I can't think of a time when they didn't announce it in some way (slip/announcement/board).
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