Is it legal for landlord to place hidden cameras without the tenant knowing?

It is possible to put a pinhole camera anywhere in the wall or ceiling, and the only way to find it would be to tear all the drywall out. If you don't find it after doing that, then most likely your parents have surgically embedded a device in your skull that is tracking your thoughts at this very second. Every time your ear itches, that is the device uploading its data to your parents' secret bedroom computer.