When a child is examined in a doctors office and they have unusual bruising, there is also no evidence directly pointing to the parents. Even if the child is asked, they will likely initially protect the parents, even if they are being abused, because they likely fear them.
Now, chances are, a mandatory reporter is going to investigate this bruising further, because of the concern it causes them and a need to find out the source. It may be, the child plays rough and got the bruising that way, or it could be, the child is being abused and you can't get to the bottom of it, without further investigation.
Same applies to young child presenting themselves (or parents saying the child should be called as the opposite gender). It would cause some concern and need to be investigated further.