Are the lil black boxes available for cyrpto! I seriously doubt even the FCC can crack freqeuncy hop on a radio with a fill in it?
Single channel plain text most likely if they moniter those wave lengths, but would they think its military chatter or civys having fun? Also they would have to know the time to sync theirs to.!
To join a net you'd need the unique NET ID, usually be within a second of their fill time, have the same hopset etc. -- not something anyone would be able to guess randomly. A spectrum analyzer could tell you which frequencies have hop activity on them but still wouldn't tell you which frequencies belonged to which hopset/net in an area with multiple radios and radio nets operating. I'm sure dedicated devices exist for such a purpose out there in espionage and counter-espionage world, but this is probably where it would be wise to shut up.
People get confused a bit about the purposes of hopping in SINCGARS to begin with. There's two levels of security in SINCGARS and similar radios: TSEC and COMSEC. TSEC is "Transmission Security": mainly, resistance to jamming and ensuring your transmission goes from A to B with the least chance of being intercepted by unauthorized C, D and E. It's not intended to provide any level of secrecy or high-level security to WHAT is being transmitted though. That's where COMSEC comes in.
COMSEC is "Communications Security" which, for want of a better term, scrambles what is being transmitted. We used to do that on a single wideband voice channel by hooking up something like a KY-57 to a single-channel radio like an RT-524 or prick 77 which gave you scrambling of what's being said - but being carried on a single channel made it very prone to squelch capture and other jamming measures. ("Jamming" = "Electronic Countermeasures" -- SINCGARS employs the hopping as "ECCM" which is "Electronic Counter-Countermeasures"; as in, that which counteracts the effects of jamming.)
Would anyone like FCC know the difference between an authorized user of some voice channel or not - especially if proper voice procedure were being followed? I wouldn't know. But one official type once told me in a wink-wink, nudge-nudge fashion that you'd pretty much be left alone "if you don't make a nuisance of yourselves..." But that's what HE said in one jurisdiction, where the one in charge of your local mob might hold a very different opinion (and confiscate your equipment - which they can do too - especially if they're 21 years old and embarking on a new career they frequently mistake for a Mission From God.)
Good radio users of everything from CB to FRS to Marine VHF to their assigned commercial channels on logging roads etc. always listen out for awhile on the channel to make sure they're not interfering with anyone else and "making a nuisance of themselves." It is also handy to have a scanner book or favourite website covering the frequencies in your area to see exactly what is used legally and by whom. For example, when you're setting up an ex involving comms you're supposed to see if there's any likelihood of interference with civilian comms in or near the frequency set you're supposed to draw channels from, and pick the ones with the least chance of interfering. One individual didn't do that once and was responsible for 35 watts jamming the local McDonalds drive-thru with CP traffic back in the VRC-12 days. There's the very definition of "making a nuisance of yourselves..."