A cage, travelcage, "sickcage" (a smaller cage you can put your rats in when ill/hurt and need to rest), hammocks/nests/places to sleep, a place (or your whole aparment/house) to give them time to run free and exercise. I can't think of anything more for now.. (maybe this wasn't what you was thinking of)
Outside you mean? I always close my windows and get burried in sweat actually. I have mine running in the living room and most of them go back into their cage when they've had enough. Some just want to be outside the cage 24/7
#2 a lot of rats would probably be pretty curious if a window was open. It has happened that rats have gone missing. Even if the owner was living higher up. They are very smart animals. And often more curious than they should be. Do think more tame rats would have more risk for walking away. Often more tame rats that have more guts to even explore more. Probably it would seek its way back pretty soon. There they are a lot like a labrador. They often like the owner and wants to be near. But rats can be easily spooked. If that happen when they are outside with no owner. The risk is bigger that they run and hide. Loose the way back. Same risk when the owner is with the rats outside. But I think they would try to run and hide on the owner if they could. I make and sell a harness just for rats. To make it safer when they are outside. Gotten pretty popular in Sweden and countries around.
But that is one thing a rat owner need to check when getting rats. Holes in the kitchen, under wardrobes. Holes like that usually don't lead more than empty space underneath. More annoying to get them out. When it comes to a ventilator, floor drain, and open windows. You might have a lot of problems if you neglect to make any safety before letting the rats run free in that space. I do recommend all new rat owners to check and make sure there are no holes or ventilations they could get out from. Better be safe than sorry. Rats can escape a cage. So have the room or house safer from the beginning is a smart thing to do.
I had a rescue-female (only a couple months old) that never came back.. I would guess it depends on your relationship kind off. My rats now always go back into the cage even if they have the whole apartment a couple of hours.