Perhaps it was a ghost. Perhaps it was an extradimensional elf. Perhaps, unbeknownst to you, there happened to be someone lounging or housed nearby who heard or saw your guitar and was genuinely inspired and decided to break their flute out, too, for an impromptu jam but then fled from sight out of stage fright. Perhaps a clique of band geeks was getting surreptitiously baked and decided to troll you. If it did happen to be the latter, man, lol, did they get you good.