The police can't demand that a person 'suspiciously' hanging around answer questions or provide ID.  Reasonable grounds does not arise from refusing to answer the questions or provide the ID.  Your example of throwing something in the shrubbery when police approach might just be those grounds,  although I think the police' first response would be to look in the shrubbery, don't you?

The reasonable grounds for insisting on ID or answers is set out by the SCC in the Mann case.  That's what police  have to articulate.