What happened to a prize captured after peace

Was a captured prize still valid if the captors discovered that the war had already ended?

Usually yes if neither side knew peace had been declared and the captor could prove it had acted under wartime orders without receiving the news.

Peace did not reach every ship at the moment a treaty was signed. A captain who genuinely remained unaware could still be acting under lawful wartime orders. The prize court therefore examined the ship's correspondence and log for contact with vessels or authorities that might have delivered the news. A continuous period at sea without such contact could establish ignorance and preserve the prize claim.