I got my first ever "This is not a boarding pass. You are checked-in, but we are unable to provide any boarding passes for your itinerary via united.com." document on a PURELY DOMESTIC trip today. Will edit this post and update this thread on whether or not this means SSSS, what the experience is like (ex-SFO), etc.

Edit: Yes, this is an SSSS on a domestic trip. This is new for me!

I asked a check-in agent at SFO to explain it — "hey, it looks like they are spelling PreCheck differently today, can you explain?" — and she said, well, that's too bad, it's going to be slow going for you on tomorrow's flight. Gee, thanks . She was happy to try to re-enter my Known Traveler # (since I happen to be carrying my Global Entry card) and reprinted the BP with no change (still SSSS). She said, "yeah, this sometimes randomly happens to employee travelers, too. Sorry."

Now I gotta repack my bags to stow the loose liquids that are TSA Pre acceptable but not SSSS certified, and probably check my bag (I am at the tail end of a cold and am carrying some non-prescription cough medicine ≥ 100 mL). Welp.

Edit: The SSSS experience at SFO was — presented my boarding pass and ID (and Global Entry card, to be petulant) to the TSA screener. Screener set aside my BP and ID and called for a supervisor. Supervisor led me to the front of the X-ray queue, had me go through metal detector, called another screener to dig into my carry-on bags and unpack everything and swab everything, and gave me a pat-down while I watched my bag get dismantled. This was fast, efficient, and everyone was very courteous. Only took about an extra five minutes versus the normal Pre-Check process. There did need to be at least two extra TSA screeners available to handle just me for a while, so I can imagine this would be a slower process if I had come at a more busy time. (I showed up early and during a relative lull out of an abundance of cautious). Hopefully this was "random" ??? and doesn't keep happening.