Has she recently had a new sibling as sometimes children do this when they see their new baby getting plenty of attention. I don't think its a big concern, dont encourage it but dont be overly worried by it, let it take its course I have heard many children doing this it is a form of endearment in my opinion , so its not good or bad .I thinks its right that it does denote a caring attitude on her part..
Well, it means she's getting rewarded or feels more comfortable with you acting like a helpless baby. It doesn't denote security in my experience - exactly the opposite.
If you want to stop it, stop responding to her when she speaks like a baby. Just say calmly that you will talk to her when she uses her big girl voice, and ignore speech that is babytalk. Like you literally can't hear it.
Best wishes. Sorry to sound so harsh but somehow, this baby behavior just flies all over me, grates on my very last nerve.