I find it hard to answer your question because that's a hypothetical situation.
If someone shows up at the border who, according to the agency of another country, presents a threat, an alarm is sounded somewhere. Border Services generally then communicates with the secret services or the RCMP to check and see whether the individual should be arrested, investigated, and so on. Measures are taken, but the law must be complied with. We can't just detain anyone at any time. Border Services will therefore take action.
Our role is not to intervene at that point. However, once measures have been taken, we will determine whether the act has been complied with. The departments currently have mechanisms designed to help them report national security threats. It is up to the organization to make that determination.