What I said was "If", not to imply that someone was lying but to suggest that events might not have occured as they were perceived, As you suggested an extra sign may have inadvertently been laid down. A sign may have fallen off a neighboring table a passerby may have picked it up and mistakenly placed it on the wrong table. Any number of things could have happened, or it is possible that someone did intentionally place a sign on this person's table without permission (that isn't acceptable if that is what happened).