CO2 pressure change at colder temps

Bought a keg system a few weeks back and used it for the first time yesterday. When I first attached my disconnects to the keg and pressurized it, it was at room temperature (70 deg). My regulator gauge read that I had 800 PSI remaining in the tank and the keg pressure was 15. I then put it in my kegerator (42 deg) overnight and in the morning the regulator read 500 PSI remaining in the tank and 12 PSI in the Keg. Would the change in temperature account for this loss of PSI or do I have a leak in the keg? Whats the best way to check for a leak when the keg is already filled? I assumed the PSI would decrease slightly, but going from 800 to 500 seemed a bit too much. Any input would be greatly appreciated.

*The CO2 tank was filled a few hours before I racked from the secondary

It is normal for your gage to read 500psi when it's sitting in the cold.

With CO2 the gauge doesn't tell you what's "left in the tank" really. It will stay at a constant pressure (at a particular temperature) until the tank runs out of liquid CO2 and then drop rapidly. If you had a leak bad enough to start dropping your tank pressure at all within that short a period of time, it would already be practically empty in the time it took you to write your question.

500PSI is quite normal for fridge temps, as is 800 for room temp. Sounds like you're fine.

It's still a good idea to check for leaks no matter what - some starsan or soapy water in a spray bottle makes it really easy, just spray it on any area there could possibly be a leak, and look for bubbles.