Hi Neil
I would put myself in the hardnose camp here, assuming you inspected the unit after it was vacated (was empty). If you did and it was, there could not have been an invitation, therefore this person is trespassing, in my opinion. Regardless of this person having the key, which presumably should have been accounted for and surrendered at the time of vacating by the tenant (unauthorized copy? - reason enough for a `key control` approach).
(Begs the question of whether you have a basis for a claim against the former tenant, especially if damage or lost income arises - ask a lawyer).
If this person occupied prior to the tenant leaving, or you can`t demonstrate it was empty, you may have a different situation - out of my realm of knowledge or experience, and most of the following can`t be applied. It becomes an improper transfer or sublet or some such thing, I would think.
If I understood what you told us here; upon discovery of this person you informed them that they did not have any right to be there, that they were to leave immediately, with all belongings, and that you would be coming back to ensure they had complied.
In my limited knowledge of the law, in most situations where you give people a timeline in which to comply with something like this it will be deemed that you have given them permission to carry on doing whatever they are doing, at least for that period. You will have legitimized their occupancy, in this case. Then it becomes a battle over the length of time, reasonable extensions, notice, etc. Similar with NSF cheques, if you give them time to replace it then you have in effect granted credit, always make your request for immediate replacement - you can be a little more lax (read vague) on your follow through.
I like the suggestions of contacting the police and suggest you arrange that they meet you at the complex, as a trespass, without going into a lot of details, if they will attend. Similarly put a security firm on short notice. I would not leave the squatter alone if you manage to get their agreement to leave - immediately (if they say it will take some time to complete just respond - `then please get started`) Their alternatives are not your problem - suggest self storage and a motel, if they don`t have money for that how were they going to pay you!
You could ask the police to wait down the hall or outside to give you some time to assess the degree of compliance, or if it can be done discretely, without direct contact - do it before you call them. Don`t wait until Tuesday, unless you have to, and try and give yourself the day for the process.
I would not take this person as a tenant, they have demonstrated that they are not truly suitable. This is not a case where they accidently moved into the unit across the hall from where you had agreed for them to be, or some such thing. They did not call before hand, didn`t offer to sign an lease and then shower you with cash when you appeared, etc. In all likelihood they know the system and you are about to enter into a battle - arm yourself accordingly.
Take a camera, and a witness with you. Photograph and inspect everything, all the walls and ceilings, windows doors, closets, even inside the toilet tanks and flush, run all faucets and appliances, to ensure proper operation, take all your checklists and do more.
Better to assume and prepare for the worst and be pleasantly or even midly surprised.
If you are somehow forced into the situation of their continued occupancy, get whatever money you can, - and look to the advice of others, regarding a short term lease etc, and/or starting the eviction process. Inspect thereafter, as often as is reasonable under the Act.
All easier said than done. Good luck, Brad