My mother has mild dementia and is in assisted living. They leave the front desk at 8:30 pm and anyone can walk outside. Is this allowed?

At the point where Mom isn't safe to know when it is safe to leave then ALF isn't the right place. Some now have locked cottages or units and that is a sort of halfway between ALF and MC (memory care). If you know that Mom isn't safe should she leave the facility, and if you fear she is too confused to know whether it is safe to leave or not, it is time to talk with the facility about another level of care. The very sad thing is that without an inbetween care such as locked cottages the cost goes up greatly for memory care due to staffing needs.

AL is not the same thing as a nursing home or memory care facility.
The residents are free to go as they please. There is supposed to be someone to let them back in if they are going to be out past desk hours but the person is supposed to let someone know they will be back. I worked in a lovely AL years back and every resident had to be accounted for after the desk closed which was around 8:30 at my place.
There is supposed to be night staff who do regular security rounds. They check on all the residents periodically, mind the doors, and make sure all is well. If the AL your mom is in doesn't have this kind of staff, someone should have a word with their administration because they're supposed to.

Sami, AL is not prison, people can come and go as they please.

If mom is at risk of wandering, AL is probably not the correct level of care. You say she hasn't done this yet, so what is your concern? Is she starting to have behaviors that lead you to be concerned?

My grannie was in a wheelchair and she was a flight risk from the NH. They had to put an ankle bracelet on her so they could catch her if she took flight.

She was in a room that was far from the front door, only door that actually exited the property but, still managed to go missing a couple of times :-( fortunately, they knew right away that Elvis had left the building :-)

I would like to encourage you to not borrow trouble. Deal with things as the become problems or you will wear yourself to a frazzle trying to stay ahead of a very unpredictable disease. She may never become a wanderer.

I would get with the ED and find out why you can't get a response from the emergency buzzer. That is a concern for safety.

Assisted Living is not a lock-up facility. Residents are free to come and go as they wish.

When I moved my mom (who has Alzheimer's, mid-stage) to AL a year ago, they told me that if she ever wandered, she would have to move to their locked Memory Care floor.

Mom and her friend in AL would take walks around the neighborhood. After a few months, they started getting lost. Each time, they were able to figure out how to call my best friend for help. (Mom didn't want me to know what had happened, but of course my friend told me right away.) We would find them and bring them back home.

After about the third time they got lost, the AL staff realized that telling my mom and her friend not to go out on walks wasn't working because they couldn't remember that they weren't supposed to go. So the staff set up the medical call buttons my mom and her friend wear so that an alarm is triggered whenever either of them tries to exit the building. I only found out when I picked Mom up for a doctor appointment, and the concierge told me they have to keep her call button so she can leave the building. They have a set of hooks behind the counter where they keep the pendants for the residents who leave the premises for appointments, etc.

Mom's cognition continues to decline, but she still lives in AL, which has exit doors everywhere, and she hasn't left the building without a staff member or family member since the change to the call pendant was made. I do not know if this sort of arrangement is common in ALs, but it's working for now with my mom. I'm grateful that this technology is available, since it gives Mom the maximum "independence" she can handle while still keeping her safe.

Sami1966: Imho, that is standard protocol for an assisted living, e.g. residents are allowed to come and go. Perhaps your mother will require a higher level of care at some point, which would be a secured facility.

Sami1966, back in April you had mentioned that your Mom cannot remember back 5 minutes ago. If that was the case, I was surprised that the senior facilities where she currently lives accepted her for "Assisted Living". https://www.agingcare.com/questions/mom-is-in-denial-that-she-has-dementia-what-to-do-474472.htm

My own Dad moved into Independent Living in a senior facility. Much to my surprise later down the road the Adm had a meeting with me where they suggested it was time for Dad to move to their Memory Care section, as Dad was found on numerous occasions trying to leave the building at night, and not coming to the dining room for his dinner.

Dad always acted pretty sharp for his age whenever I visited. Apparently he was "showtiming" when I was there. And now he was "sundowning". So we quickly choose a room in Memory Care for Dad. It was easy to move Dad as I told Dad a "therapeutic fib" that this new room was less expensive and Dad was always trying to save money :)

I'm in Minnesota, and what you describe is exactly how assisted living is supposed to work.
Assisted living is just that: assisted. It's meant for seniors (or others) who need occasional assistance with things like cooking or cleaning. It's designed for independent adults who just aren't able to complete some household or personal tasks. Just like in my apartment building, you are free to leave whenever you want-- it's not a jail, and it's not designed to be.
What you are in need of is memory care. It may be an option in your mother's building and it may not be. MC units are locked and staffed 24/7. We specifically chose a building with MC for my parents when my dad had mild Alzheimers, knowing that it would progress and he would eventually need 24/7 MC.
The plan was that when my dad required MC, mom would still be in the same building and be able to see him every day.
Unfortunately (or fortunately), his disease progressed very quickly, and he died before we could move him.
Once he was diagnosed, he could never have lived on his own in AL. If my mom turned her back, he would dump hot coffee or wander off, or anything else to put himself in danger.
I'm sorry for what you're going through. I can definitely relate. I felt like I went through 3 years of hell, mainly trying to help my mother survive my father's disease. There are a lot of us out here, so keeping reaching out and be sure to take care of yourself.

It will soon become time for memory care. I suggest you start looking around without your mom first

People in assisted living can come and go as they please. Many of them even own cars and drove themselves. I think you are confusing assisted living with memory care. In a memory care all of the doors are alarmed and people are not allowed to wander.