Leaves you are propagating can definitely be kept outside! If it is currently warm where you are living (coldest temps above 40F), you can move them outside.
There are a couple of things you’ll want to watch for.
First, you’ll want to make sure that the temperature doesn’t get too low at night. You definitely don’t want them to freeze!
You’ll also want to make sure they don’t dry out too much, but also don’t get too wet. Be sure the leaves are laying on well-draining soil. If you’re living in a naturally humid or rainy environment, this is crucial.
If the soil stays too wet for too long, your leaves will most likely rot. However, if they don’t get much rain or water, it will be difficult for roots to form.
You don’t need to keep your propagating leaves covered and, in fact, they won’t do well under a plastic covering.
Be sure to keep them out of direct sun for long periods of time. The leaves need light, but too much can cause sunburn, just as it does in adult plants.
Good luck! If you’d like more information on propagating, be sure to check out my ebook, The Secrets to Propagating Succulents.