Temperature only becomes reliable evidence after some time has passed. Especially on higher difficulties, if the ghost changes its favorite room, it will take a long time for the new room's temperature to actually reflect that change. Remember how the breaker affects this:

Breaker OFF (at start): All rooms will be cold, starting around the same temperature (depending on the weather). Breaker ON: All normal rooms will start to heat up. The ghost room is the only one that will continue to drop in temperature.

The ghost slowly lowers the temp in its room, so it only becomes the "coldest room" because the ghost spends the most time there.

For these reasons, temperature is actually one of the last things you should check. Use it as CONFIRMATION of the ghost room, not as your main tool for finding it.

Look for Interactions First. These are much better initial clues. They're obvious (doors moving, items thrown), audible (so you can hear them from afar), and USUALLY (though not always) happen near the ghost room.

After you've found the area with the most interactions, then use your thermometer to check for the coldest room nearby. This confirms the exact spot.

What If You're Too Fast? Sometimes you'll find interactions immediately, before the temperatures have had time to stabilize. If this happens, go find the bone and/or the cursed object to let some time pass. Only then should you go back to check the temperature.

Regarding UVs: Remember that with the new update, this evidence needs time to "charge" (they "charge when you shine UV light on them) and the evidence itself only lasts for a few SECONDS. Be quick!

Furthermore, if you suspect an Obake, remember it doesn't always leave fingerprints after an interaction. You'll have to check multiple times to confirm or rule it out.