I don't know if she appears on the beach, but you could check, just in case.
I don't know if this is a rule or not, but in my experience, Sahara tends to stick to one area of your town for some time. You could check each area at a time, without quitting or reloading. The map is relatively small so it shouldn't take long. If you have areas with a lot of trees or buildings she could hide behind, press the down button to see through them.
She always finds a stand of trees in my town to hide in The megaphone should locate her. I've never seen her on the beach, and she won't follow you down to the beach. I don't think she's there. She may be a character that disappears when your gates are open, but I've never actually thought about that till your post.
- - - Post Merge - - -
Yes, I'm finding online that she does indeed disappear if you have your gates open.
I found out that after you reset and load up the game again, Saharah isn't there anymore. But if you save and continue and load up the game, she will appear again.
Well, I think if you loaded with an existing character and the game saved as you loaded then she should be there even if you reset. But if you loaded with a new file, I think Sahara's appearance is random, so probably not a glitch that she was gone.