What would happen if the horses got out? Would they just wander but stay close? Would they act like a pack and run off together? Would they bolt?
It depends on a lot of factors. I will often let one horse outside of the fence, free to graze (I'm sure you've seen it in a few of my photos) Cheyenne is especially good with this. I've been doing this for years and they more or less know the routine. Stay close or you get put back. And they really don't want to be put back. They, of course, love the grass. Alone, they will usually stay near the 'pack' still in the corral. Let another one out.....and especially the third, and now we have a herd on the loose. lol There is a ton of property around us where they can graze but also a ton of property where they can not be easily seen. If one horse jumped that broken fence, I'm sure the others would follow. Herd mentality. 'You going? So am I! Wait up!' lol Once out, grazing on grass would be the first thought. But if one of them decided to bolt, the others would follow. They would all stay together, as a herd does. But I can see them 'losing sight' of the fact that 'this is where we live'. Like a herd in the wild, they would most likely graze freely and keep moving without thought of 'staying put'. Now, if only one got out, they would would most likely stay close to the others.
A few quick stories to back this up....One time, years ago, I had Stilts out free with the 2 girls still in the corral. He suddenly took off towards the back where someone had recently gotten horses. My two neighbors here still had none. There were no fences up next door and he just ran through that prairie at a diagonal to our house. I jumped on my ATV and gave chase, while the mares in the corral screamed for him! We ended up making a big circle because he listened to the mares screaming and came running back, with me close behind. He, of course, got put back in the corral that day.
When I first bought Cheyenne and had no other horses, she was out grazing loose while I was on the lawn mower. There were no other horses in the area at the time. The white pasture fence next door was not there and it was only an open prairie, similar to ours. We had only just begun to clear the land and plant grass. The neighbor on the other side had no horses either, so Cheyenne was all alone and was a good girl about staying close, even as I mowed. As she grazed closer to the edge of our property (but close to me mowing) I would stop and say, "Cheyenne!!!"

She would look at me and turn a bit to stay in the direction I wanted. Then one time she was getting too close to where I didn't want her to go. I stopped moving the mower, called her again and when she looked at me, I threw my arm and pointed in the other direction (back to our property) and she actually turned around and went in that direction!

It actually surprised me too!! lol I start mowing again and as I turn around, a friend of mine was standing by the house, obviously trying to get my attention through the noise of the mower. I yelled, "Did you frickin see that??!!? LOL What a good girl!
But then another incident showed the mentality of the herd......Cheyenne was alone in the corral when my the neighbor who used to live next door (and had recently gotten a horse) told me that both her horse and mine were across the back field in someone's front yard!

No! Couldn't be. From the house I could see the corral gate was closed! But when I went out there....no horse.

We had surmised that when her horse got loose (I forgot how that even happened) he ran past Cheyenne and mob mania kicked in! lol "What's happening??? Where ya running to? WAIT FOR ME!!!" And she jumped the corral fence! No other explanation for it. The gate was securely latched and jumping the fence was the only way she could have gotten out. We each grabbed a bridle and had her son drive us over there where we found them being chummy with some other horses.

We easily caught them and jumped on and rode them home. Bad little ponies.
