Is it ok to leave food out for Mom while I’m out?

don leave on table if it needs to be refrigerated or she will get sick. You can leave it prepared & tell her it’s in fridge on top shelf in front. Then call to remind her about it. Hugs 🤗

Asking the next door neighbors or those across the street to keep an eye out for her for that day, and posting hand written signs on the doors not to go out, and a couple of signs near her favorite chair, her kitchen, etc. that you are at a conference and will be back at O'clock, plus food arranged as others have suggested, ought to work.
there are no absolutely safe guarantees of safety in eldercare, sadly.
I worked as a nurse in a SNF - we had a lady who loved to walk...and was standing at the counter with the CNAs listening to report (meaningless to her) and suddenly fell backwards to the floor! In front of 2 nurses and 8 CNAs. She didn't have a fall history....until that day.
Take care.

Why don't you try a "dry run", leaving for a brief time and call her to tell her that lunch is in the frig. Show her a lunch plate for several days, when you're still there, so she becomes familiar with the plate and location in the frig.

Why not leave her a cold plate in the refrigerator? My sister, daughter and myself take turns caring for my dad in the evenings. On the rare occassion that we are unable to make it, we will drop off a cold plate. Usually a plate with some grapes, chicken salad, picl;es, cheese and crackers. Cover it with plastic wrap. Call to remind her that her dinner is in the fridge. My father enjoys it!

Yes. It's fine. You can leave a covered plate or basket with room-temperature foods in it for her to snack on, and/or you can leave her something in the fridge and call to remind her to fetch it at, say, 4pm or whatever time she normally does eat.

What is it exactly that you fear might happen?

I think, as some others said, the food isn't really the problem. You can always leave out safe things like fruits and veggies, can of ensure, and etc.
You say "mild dementia" and only you can evaluate your Mom for that. You are now adding your concern, though you feel it unlikely, of a break-in. My concern is always wandering out and fire inside.
Given that you do have concerns I would ask friends, neighbors, etc if they have a trusted baby sitter who could simply be "present" for those hours. I wouldn't myself be able to relax over that 8 hours if your choice has already been typically never to do this, and this is a one time thing.
Costly, perhaps, but much more safe and comforting for you.

If this is a question about food spoilage, well, it depends on the food. My experience is that most older people keep the indoor temp. higher than others would, so this might be a concern.

A PB&J sandwich will keep all day without refrigeration, although the PB may become a it runny. An apple, a banana, a cutup orange: these will keep for hours or more without difficulty.

But something with mayo in it should not be left out for four hours. Nor should meets (even lunchmeats, although these are full of salt and preservatives). American cheese slices with mustard would work, however.

And, if you think there might be mice in the house (it happens) you'll need to improvise a mouse-proof container. A plastic thing with a snap-on lid will work for this, but you'd have to be sure she can open it.

Although if there's concern that she might fall, or there might be fire or other emergency (yes, that happens too) you might look into a PERS ("personal emergency response system" that is, "I fell down and I can't get up") if you think she could and would use it in an emergency. Although these are cheap to acquire they have a monthly fee.

A cheaper alternative is a phone app such as "Big Red Panic Button" for Android. This will NOT call 9-1-1 but you can set it up to send a canned text to a list of people (such as you) who can then respond. Again, if she would be able to use it.

Maybe an early lunch or large snack before you leave, then a phone call reminder that her dinner is on the counter? And even if you come home to discover she hasn't eaten or rummaged around and ate something else it's not a big deal, she'll be fine for one day.

AsianDaughter, it would be ok as long as your Mom doesn't have issues while eating, such as eating too fast, swallowing wrong, or coughing due to food/water going down the wrong pipe.

I don’t think leaving food out is the issue unless it’s something perishable that could go bad sitting out. The bigger question to me is, is mom safe alone? If there were a fire or break in would she know what to do and be able to carry it out? Might she wander when alone, it’s sadly so common, and if being alone throws her routine it could happen

You want to make sure that whatever you leave isn't going to spoil by being left out or find a way to keep it cold.

I don't see a problem leaving items out to ensure she eats.