When we enter a store, EVERYONE knows it...My daughter is SO loud! We've experienced meltdowns everywhere. When my hubby and I get brave and venture to a restaurant with her, we order our food and ask for it to be put in a to go container instead of on a plate, along with check. That way, when the meltdown happens, we are ready to run! We do take her out a lot, though, trying to teach her appropriate behavior in public. She doesn't get left home with a sitter.
One thing she does, which is actually kind of cute to me, probably annoying to those who haven't walked a day in my shoes, is say "hi" to everyone. She will not stop saying "hi" to someone until they have said it back to her. She will even yell it down an aisle if they hadn't said a return "hi" to her. She doesn't realize just how loud she is. Our behavior therapist has worked with her on using a quieter voice, but no luck with any of the suggestions. So, if you are ever out and a child says "hi" to you, please return the polite gesture:-) She LOVES it!