> Each device has a test mode that will communicate with the SARSAT satellite network without sending an alert.

That explanation is factually incorrect, then. There currently is no "test" flag, nor is there the required infrastructure to check if your test alert went through:

https://www.sarsat.noaa.gov/emergency_beacon-testing/

So this "self-test" feature can't be communicating with actual satellites. No idea what it actually does, but it's definitely not an end-to-end test.

Perhaps the emergency communication with satellites is a handshake, and the "test" mode for the beacons simply doesn't complete the handshake?

Possibly (the page I linked even mentions a "test" type of signal), but given that there is no return link for most PLBs and no "list of recent successfully received test transmissions" online, I don't understand how one would actually verify success.

This policy document mentions that the "self-test" feature actually does not communicate with satellites at all, and also mentions a "test frame" that is discarded by satellites, as well as a "test protocol" that is forwarded by satellites, but discarded by the ground segment: https://www.sarsat.noaa.gov/wp-content/uploads/POL-MCC-051-v...

So it seems like both the "self-test" and the "test frame" don't tell you a lot about how well your beacon would work in case of an actual emergency (unless you have equipment that can receive and interpret that signal), and live testing involves a lot of paperwork.