No, she gets spotted by Edelgard before she could.

As the Black Eagles left their cottages to go to the beach, Petra tried to hide from them before she got seen, so that she could go get her swimsuit from Dorothea later. To do that, she looked around her zone to see any good hiding spots for her, be it palm trees, natural rocks, or bushes.

At the end, she chosed to hide behind a palm tree next to the cottages, so that she could move quickly from it to Dorothea's cottage as soon as everyone left. But unfortunately for Petra, however, her plan failed right at the start, as she got spotted by someone just when as she was about to hide.

"Petra, what are you doing", the voice said in a suprised tone upon seeing her.

Upon hearing the voice, Petra then reluctantly turned around to see who said, and it was none other than her house leader, Edelgard, wearing her white and red swimsuit.

"Ohh, hello, Edelgard, how are you doing ?", replied Petra in a nervous tone while trying her best to cover herself with the tree.

"Nothing, now tell me, what are you doing being naked here ?!", Edelgard replied concerned.

Due to her relative still lack of knowledge of the language of Fódlan, and nervousness, Petra couldn't manage to come up with the words to explain the matter to her right now. But fortunately for her, Dorothea came to explain the situation for her.

"Don't worry Eddie, it's just that back in Brigid they like to swim in the nude", explained Dorothea as she arrived.

"Yes, i used to swim nude when i was younger here", Petra added, not as nervous as before, but still somewhat. "That's why i don't have a swimsuit of my own", she further added.

"Ahh, i get, i guess", replied Edelgard more undertanding of the situation, "Well, is not to shame you or anything, Petra, but I don't think it's a good idea for you to swim nude today, specially with the guys here", she argued.

"Yes, that's why Dorothea offered to give me a spare swimsuit", she replied.

"Well, go get your swimsuit then, i'll be at the beach if you need me, but bear in mind that if the boys see you like are right now, that's on you", Edelgard replied and warned, before leaving Petra and Dorothea alone.

"Well, that was...awkward", commented Petra a bit red as she realized that her house leader had just saw her naked just now.

"That doesn't matter now, let's go get you a swimsuit!", exclamed Dorothea, upon which, she and Petra went back to the plan of getting a swimsuit for her, yet, for how things are looking right now, it looks like Edelgard won't be the last person that would be seeing Petra naked today.