Sources close to the new couple tell TMZ ... Holly came into Zak's Haunted Museum in Las Vegas early last month and the sparks were flying, so they started talking romantically. We're told things picked up pretty quickly, and they started dating.

The "Ghost Adventures" star and former Playboy model have been keeping their relationship low-key ... but eagle-eyed fans have noticed similarities on the celebs' Instagram stories, which hinted at something between them.

Our sources say Zak and Holly had been introduced before she strolled into his museum in May, but neither was single during that initial interaction.

Holly famously dated Hugh Hefner -- in her "Girls Next Door" era -- before marrying Pasquale Rotella in 2013 at a Disneyland wedding ceremony ... but last September, Pasquale filed to divorce Holly.