Do local police surveillance vans, along with government agency surveillance vans, emit a visible WiFi signal?

No, they do not. Any communications they have would not have broadcasted SSID's, or, the name you look for when you're selecting a wireless network. 

Of course they do, but usually they dont advertise that they are FBI, their ssid would usually be titled as free wifi, and its usually a trap because criminals like accessing free wifi hotspots to do their illegal business.

 

 

 

My above statement is not true.

There's actually another issue with this, law enforcement, military, and other government organizations that handle sensitive data do not use WiFi as most people know it. Instead, LAN/WANs are used that have secure VPN tunnels that route back to the agency. These provide fax, phone, video, and other media capabilities. 

In general, law enforcement is sending and receiving information on an ever increasing level, and much of this information is sensitive, and must be kept secure, not only for privacy, but, also for the safety of then men and women whom we refer to as first responders. Some of this includes information that is labled LES, or, Law Enforcement Sensitive. 

The information that is transmitted in a "FBI surveillance van" is not going to be public information, the government is going to do a lot to ensure that critical information doesn't fall into the wrong hands. If you search google for "FBI Surveillance Van"  you'll find a lot of stories about people who have nothing better to do, and, find humor in labeling their WiFi something stupid. 

In reality, the US government has extremely strict communication policies, to the point that many, if not most, government agencies can't even use WiFi because their government issued laptops have the WiFi either physically disabled (meaning that the end user wouldn't be able to turn it on, even if they wanted to) or not present on the computer.  When, on the rare occasion, they do use WANs, or LANs, they are behind such hardened firewalls and security appliances and encryption, that it is practically impossible to defeat the security. 

They start by not even letting you know that a network is even there. They're not going to advertise their presence and make themselves a slightly easier target.