[RinaseaofDs]

Huge monolithic buildings that are heavily guarded with heavy fencing and very few windows.

[daltec]

Thanks! Are they AT&T? Although, hard to tell these days. On rare occasions I have seen heavily armed guards (body armor, helmets, SMGs) outside of otherwise perfectly nondescript "storefront" types of buildings. Others are as you describe.

But your observation about no or very few windows is key -- they could very well be COs for your local telephone carrier. Verizon COs in the northern VA area are usually small affairs, since there are so many of them. But none of them have any windows, to speak of.