That doesn't quite help if you don't know the name of the room you were in, only that it started with a P. If it were in alphabetical order you could find it quickly just by scrolling to the P section.
You can type "p" on the searchbox then and see which chatroom has what amount of people, clicking on "next result" everytime you want to check another one starting with "p". If you still can't find what you're looking for, then even if you could see all chatrooms starting with p you wouldn't find anything.