Hugh,
Mr. TV asked if government agencies monitored some frequencies 24/7. I am an Army Aviator and can tell you that 121.5 VHF is monitored by every aircraft in the U.S. (not just military). The VHF radios in aircraft are designed to automatically receive 121.5, and it will broadcast into the ears of the pilot regardless of currently tuned frequency. Example: I have a tower freq tuned (119.325) and if someone broadcasts over 121.5, I will hear it, but in order to respond, I would have to tune my own radio to 121.5. The same is true for aircraft with UHF radios installed (243.0). Most civilian aircraft don’t have a UHF, but all military aircraft do, and it works in the same way as the VHF.
Additionally, all Air Traffic Control (ATC) services (centers, approach/departure controls, towers, and flight service stations) are constantly monitoring 121.5, just like the aircraft are. Most ATCs also have UHF capability, so they hear 243.0 also. A flight service station (FSS) can be reached from almost anywhere in the country because of remote communications outlets (RCO’s), but in the event of SHTF with a power down event those will be gone when the battery runs out, because they are nothing more than a repeater. A FSS can usually be reached on 122.2, but these freqs can vary depending on location. Look at a VFR sectional (aviation map) for the specific freq in your area. VFR sectionals can be viewed at airnav.com or skyvector.com. There are others, but those are the two I use. Hopefully it helps. – DH