ive heard all sorts of stories, which you can find here. some say the cold makes the strings brittle and lose their life. Its mostly for extreme tempuratures, so if you're concerned you'd need a thermal bag
If you warm the strings with your hands before playing with them, you should be OK with your strings. Cold strings do have more of a tendency to snap.
I live in KL, Malaysia and the sun is very hot here. My car is parked in an open area. Not sheltered.
Thermal fluctuation is always bad. So if you car heats up during the day you should think of another place to keep your racket.
then you got a problem extreme heat and extreme cold could destroy your racket (yes not the string) I'm sure there is a thread about it, but I'm too lazy to search for it even with thermal bag, it is bad to leave your racket in your car exposed to the heat (remember when you open your car and it feels like in an oven? yes, you are going to bake your racket)
basically room temperature / stable conditions is always best for racket or strings. its common sense that extreme temperatures arnt good for anything in general.
I leave my rackets in my truck during the winter, I just leave the bag open so that they heat up on the way to the gym.
Just take your racquets out and bring them in the office. If your boss says anything, say it helps your productivity to know your racquets are safe. =)