my daughter is 19 months, and has only on rare occasions stayed under the blankets. I have noticed that if she is super tired, and falls asleep as she hits the bed, she stays under the blankets, but that doesn't happen much at all. When I go to bed, I always check on her, and put the blankets back on her. If she kicks them off than i feel she is too hot. If she is cold she can get them back on.
I try to keep footy pj's on her in the winter and during the day, i put slippers on her. She keeps the slippers on, so i don't worry about them coming off at all.
Even my 7 year old step son, will kick all the blankets off him, I figure I do that at night if I am hot, so why wouldn't they. He puts them back on when he is cold, and so do I. So I am only guessing that's what my duaghter is doing as well.