But how do you guys know that they dont meet in a coffee shop or even THAT coffee shop EVERY mission?
Because that's something only Encyclopedia Brown would do. Most freelance detectives and agents have offices-- it's what separates them from base vigilantes and gangbangers.
Harry Dresden, Sam & Max, Dirk Gently, Mack Megaton, The Hebrew Hammer, Bryan Roses and Harman Smith, Detective Conan, Ace Ventura, Adrian Monk, Scott Shelby, damn near any team in Shadowrun, Hercule Poirot, Sherlock Holmes, even Dante from Devil May Cry... they all had offices!
It's a good way to keep your documents and files together, it's a good place to meet clients and keep a hold on your employees, and it's a good place to hide your treasury and your armory if you're of the "my enemy's wallet is my client's fee" types. Frankly, I don't see why any self-respecting organization, "freelance" or no, would forgo anything close to a base. "Freelance" doesn't even mean "doesn't have a base", it simply means that they're self-employed and not playing to the whims of the police or a larger corporation.
I'd be hard-pressed to find a decent-sized organization that wouldn't have a permanent (or mobile) base. I mean, even the Winchesters operated out of their own home! The only kind of organized anti-supernatural group without some place to call home and some way to keep records of what they're doing is either a small family-run business (which SIN isn't),a cult (which SIN isn't), a group of child detectives finding some way to occupy themselves after school (which SIN isn't), or a group of violent criminals hiding out from the law (which SIN isn't).