You could always try going "You're going to have this time out, and you are going to think about why you shouldn't have gone outside without asking me. When you have an answer, you can tell me and we'll talk about it. After we've talked about it then your time out will be over."

It's giving a purpose to the time out. A 5 year old is capable of going "I shouldn't go outside because I might get lost/hurt." or something similar. You can follow up with "That's right, you could, and that would make us both very sad. When I don't know where you are, it scares me because I want you to be safe."

Then it turns the time out into a teaching moment and will help her understand consequences and emotions. If she doesn't have an answer after a while, then you can just share with her how you felt when she didn't listen and explain why that rule is in place.

And, if she has an answer quickly you could possibly bring up the "So you knew that wasn't safe, and you did it anyway? How does that make me feel?" which makes her consider your feelings. If she hasn't got an answer you can then tell her, or if she says that it makes you feel angry, you can correct her and go "No, I'm not angry. I felt scared that I didn't know where you were. How do you want me to feel?".. when she says something like that she wants you to be happy, you can go "That's right! I want us both to feel happy, and I'll feel happy knowing that you're being safe. Do you feel happy having a time out?". She'll say no to that, and you can remind her that she'll be happy if she's being safe too.

I do something similar with my class when they're misbehaving. They have to tell me how I'm feeling and how they feel knowing that I feel that way. They then have to think about how they want to feel, and how they want me to feel - and what the classroom would be like if I was feeling that way. It helps to teach them how their actions affect others, because children don't always have that understanding - it also helps them understand why rules are in place.