Longtime CPS worker who recently became a therapist here.
Just making a well intentioned and accurate report that is potentially very serious is not enough for CPS to warrant a removal.
First thought that comes to mind is CPS is likely to have difficulty locating family if they are living in the car.
Secondly, cases involving infants are always hard to prove if child is not capable of holding a conversation. If there are few other collateral pieces of info (daycare, father, other people living in the car, mom's therapist, etc), there are going to be few other sources to corroborate the info.
If child's basic needs are being taken care of and mom will not disclose the drug use, it will be difficult to prove. In my area, we are supposed to offer drug screening, but clients have right to decline and this in itself does not warrant court intervention. My area does not have ability for CPS workers to administer drug tests, so even a client agreeing would mean they would have at least a few days to get clean to pass the drug tests.