That 7 year old is way to old to be acting like this- I would keep and eye on her-
as for the things in her panties- children this age are just packrats I have seen children do this when they just want a "pocket" -it is a bit odd - but don't talk TOO MUCH- She may be getting so much input she has no idea what you are getting at-
just tell her it is not a good holding place for things as it is not as clean as a purse etc- get her a little purse or old diaper bag to carry-
In a child of this age it can be considered a learned behavior, check out who she is alone with ,anyone takes care of her, check that first, as these are rather extreme behaviors.The 7 year old could be the instigator and she has also learned this behavior, speak to her Mom and find out what is going on in their lives.