I am not sure this was the right call, but that's the least of my complaints about what happened.
For some inexplicable reason, the agency also told the aide that she should go home rather than accompanying Uncle to the ER. This is not the correct procedure. At all.
The aide is supposed to ride with Uncle to the hospital in the ambulance, and stay with him unless he is admitted. Only when he is actually admitted to the hospital is aide service supposed to be suspended. (2/15)

I have since asked the aide agency, in writing, for an explanation for why this procedure wasn't followed. They have not (yet) responded.
The ER diagnosed hyperglycemia, which they treated with insulin, which he could (and should) have been given at home, avoiding the need for a trip to the ER.
At around 4am the ER sent Uncle home in an ambulance, with no aide. The EMTs brought him into his apartment, transferred him from the gurney to his bed, and left him there. (3/15)

To reiterate: Uncle, who has 24x7 personal care aides because he cannot care for himself, was dumped into his apartment alone in the middle of the night, with no one having made any effort to resume his aide service then or at any point later in the day. (4/15)

When I woke up, I checked Uncle's account in the hospital portal and confirmed he had been sent home from the ER. I was a little surprised by this, because (a) I vaguely recalled that the aide told me, when she called to tell me he was going to the ER, that she had been told not to accompany him, and (b) surely the hospital would not have sent him home without an aide, right? (5/15)

Uncle had a doctor appointment scheduled for 11:30 today, and the medical transport would be picking him up at 10:30, so I started trying to reach him by phone at around 8:30 to confirm that he was awake, having breakfast, and getting ready to go to his appointment.
Even if he had been sleeping, someone should have been answering his phone, since the aide is supposed to do that. But no one answered any of my calls between 8:30 and 10:00. (6/15)

At 10:00 I canceled Uncle's ride and appointment and switched from "making sure Uncle makes his appointment" mode to "Uncle is probably trapped in his bed in his apartment alone and in God knows what condition and I need to save him" mode.
[While all this is happening I'm not at home, I'm down in Maryland helping with my parents' care while my sister who lives near them is away for work, so I'm driving my dad around while desperately making phone calls trying to make sure Uncle's OK.] (7/15)

Next, I called Uncle's friend I know in his building and asked her to check on him. She went to the door of his apartment and found the door locked with an Instacart delivery stacked in front of it. She called back and told me, incorrectly, that this must mean he wasn't in the apartment. This just made me worry that maybe he was in the apartment and unconscious or worse (I assumed that they knocked and he didn't answer, but they didn't explicitly say they had so maybe they didn't?) (8/15)

While she was on the phone with me telling me Uncle wasn't in his apartment, I was on hold with the ER, which I'd called to ask for confirmation of if/when they sent him home. Right after she hung up, the ER picked up and confirmed that he had been sent home around 4am. I asked wtf they were thinking sending a disabled person home in the middle of the night without a caregiver, and she said she didn't know, she wasn't there when it happened. smdh (9/15)

Next, I called another friend who lives nearby and has keys to the apartment and asked him to go by the apartment and check on Uncle. He said he couldn't do that for a couple of hours since he wasn't currently at home. Another dead end.
Next I called Uncle's building's security desk and ask them to do a welfare check on him. They say they will have to ask management to do that. I say yes, please do that. They said they would and hung up, but I wasn't] convinced they meant it. (10/15)