Two incidents in Opawa and St Martins has prompted the warning.
When the residents returned they found items stolen and the offender gone.
It is too early to say if the incidents are connected, and police are keeping an open mind.
However, it is a timely reminder to keep properties secure.
"There is never any obligation to answer the door to someone you don’t know – but if you do, don’t step away and leave the door open" said Detective Constable Samuel Moore.
"Security features such as a peephole or screen door can also be used to safely assess any visitors."
"It is also a good time to have a conversation with elderly friends, family or neighbours about taking these precautions for safety in the home."
If you see any suspicious activity, call 111.
