They have transponders. But military airplanes can and sometimes do turn them off. When airplanes broadcast signals they can be detected. The military sometimes wishes to avoid this.
One of the article updates quotes from a Washington Post story:
> "The jet’s transponder, which usually helps locate the aircraft, was not working “for some reason that we haven’t yet determined,” said Jeremy Huggins, a spokesman at Joint Base Charleston. “So that’s why we put out the public request for help.”
It could activate after ejection, certain acceleration pattern (i.e. crash) or could start broadcasting only after receiving very specific signal (one-time code) on a specific frequency. There are probably plenty more options, it doesn't seem like a hard problem from engineering standpoint.
Civilian aircraft, even 2 seaters, have had ELTs for decades. They are being replaced with better systems, but they were designed to do just that: broadcast a signal on a specific frequency after a crash.
Doubtful you want to broadcast the position of the latest generation stealth fighter, however.
As mentioned, it can be silent until receiving specific signal. It can be single frequency it can be multiple, but if it's specific string of bytes or specific timing it's practically impossible to brute force when it's long enough, and when looking for the plane you can broadcast it with high intensity because it is only single use.
I don't think stealth aircrafts normally have trackers or anything broadcasting a signal. Kind of defeats the purpose...
I did read that stealth aircraft have transponders installed when operating in US airspace, so commercial radar can see them better. But it was not installed on this flight.
The article mentions that they have transponders that can be used but apparently in this case it was off or not working. They can also attach a radar reflectors when they want to be more visible to radar, and apparently the weapons configuration plays a role too:
> "Also, the jet's configuration and its avionics' operability are an issue. The F-35s wear radar reflectors when on transit flights, as well as on many training missions and some operational ones. The Marine jets often wear missile rails for AIM-9Xs, as well. But if the aircraft was in its full stealthy configuration and had avionics issues, tracking it may have been troublesome."
They have transponders that transmit their identity and location on the most modern, but those depend on being active (for some reason the one in the F-35 was not) and on there being receivers nearby to pick them up. At lower altitudes in remote locations coverage is spotty.
First, yes it would be a security risk.
Second, AirTags depend on there being a set number of iOS devices nearby to provide location data and an internet connection. You're not going to get that in a rural area.
GPS is not reliable underwater, I'd expect it to have an ELT/EPIRB but those don't work with GPS as far as I know, and the crash could have been hard enough to render it inoperable.