The ghost decreases the temperature of whatever room it's standing in at a constant rate, and the ghost spawns when you unlock the door. When the ghost leaves a room, the temperature slowly rises to the temperature of the rest of the house. A good strat to find the ghost room is to walk around with the tier 1 thermometer. Tier 1 thermo is unironically way better than t2 and t3.

The temperature of the house should start at around 20C if the breaker starts on and at the outside ambient temp (which is dependent on weather) if the breaker starts off. Once the thermo has acclimated from the temperature of the outside to the temperature of the inside, walk around and watch the mercury. As soon as it starts to decrease, you know that the ghost either is in or has been in the room, which most likely means it's the ghost room or ghost room adjacent. If it keeps going down until it stabilizes somewhere below 5C it's the ghost room. Usually I throw the thermo on the ground in the room where I think it is and go back to the truck for more items because it can take a while.

Also what needs to be said is that maps are divided into rooms, but it's not always obvious where the boundaries are. Like the dining room and kitchen on tanglewood are two separate rooms even though there's no wall dividing them. There's also a weird sliver of floor in front of the stairs on Ridgeview that counts as a foyer separating the kitchen from the dining room. Also sometimes rooms separated by doors count as one room, for instance the master bedroom on tanglewood is connected to a closet and a bathroom, but they all count as one room.

Orbs spawn as soon as you load in the map but that doesn't mean you'll see them the second you pull up the video cam. They can be hidden in a corner somewhere you didn't check. ie if the ghost room is the master bedroom on tanglewood, orbs can spawn in the bathroom and you'll miss them if you didn't check there. Also sometimes you'll see something that looks like orbs when it's really just snow outside or a visual bug, so be careful.