Hickey or rash on his neck?

My bf came from work with two marks on his neck, i think they were hickeys but he said its an allergic reaction to something. I didnt belive him because he was trying really hard to hide them from me, he said he didnt want to start a fight.i think the next day or the day after that the marks were gone, could he be telling the truth? how long it takes for hickeys to dissapear?