catchword, clue, cue, cue word, danger sign, early symptom, evidence, falling barometer, gathering clouds, high sign, hint, hot lead, intimation, key, key word, lead, omen, precursor, preliminary sign, premonitory sign, premonitory symptom, prodroma, prodrome, quarantine flag, red flag, red light, scent, skull and crossbones, spoor, storm petrel, stormy petrel, suggestion, symptom, telltale, thundercloud, thunderhead, tip-off, trace, vestige, warning sign, warning signal, whiff, yellow flag, yellow jack