For the record, I'm using FP-10 "Shooter's Choice." The same stuff I use on all my handguns. Never had a problem with this oil. It's worked great for years now. It smells like cinnamon too.
FP-10 does not protect against rust any better than 3-in-1 or other basic oil products. While that should be sufficient for a few days or more on a blued gun (the bluing actually helps retain oil for preservation), clearly it is not in your case.
Probably the simplest solution would be to switch to Weapon Shield CLP and see if that does the trick. I hope that I don't sound like a shill for this product, but it has worked well for me and I'm just trying to help. It happens to be designed by the same person who originally created FP-10, and it also smells like cinnamon.
He has improved the product in every way, including rust prevention. It performs among the best in this regard, although there are better. Eezox, for example, is serious medicine for rust-prone guns, but it doesn't seem to be as good a lube or solvent, and it smells toxic. Weapon Shield Grease is actually a better lube and preservative than either of these, so it's worth considering as well (yes, it smells a lot like FP-10, too--makes me hungry for breakfast pastry
). If you lightly coat your gun with this stuff and wipe it off with a rag, then it should be able to resist even a saltwater spray for days on end. Unfortunately, a grease is not the best product for treating your gun's bore, so you might need one of the others, in addition.
To the safety nazis: I'm not pointing a loaded gun at my girlfriend (not my wife). The pistol was cleared long before before I took any pictures, the magazine was nowhere near the gun, and I double-checked the chamber before snapping any photos. She was watching tv to the left and I had to stand under the light to get a decent indoor shot without using the flash. But I appreciate the concern fellers...
Well, OK, since you JUST checked it. Maybe this makes me a Safety Nazi, but I never point assembled guns at anybody even if I did just check because I'd rather consistently reinforce a good habit every chance I get, but that's just me.
Now I just hope that it won't start to rust on me again. I'll have to try harder to prevent it from happening again. I'm still not totally convinced that the environment inside of a car is THAT bad, but then again, maybe it is. Either way, I'm not about to find out for a second time.
You never know what sort of chemicals may be floating around that can promote the formation of rust. You can either treat the gun's finish more frequently or use one of the products I suggested above.
Just an after-thought here, but I'd rather pay an extra $100 up front for a nice treated steel finish, rather than going through all this. I don't know why they don't offer a better finish considering the fact that they know these pistols will be heavily carried in a humid environment (in the pocket). Just my 2 cents I guess.
Blued guns have been used successfully in all types of situations and environments throughout much of the history of firearms. They require more maintenance than some modern guns, and that's simply what people used to give them. Hopefully some of the newer corrosion-inhibiting products available today can make this less of a hassle.