Unauthorized occupant

[Wesley W.]

Hi folks,

Okay, I'm pretty sure my current tenant stays a lot at her boyfriend's place, and has allowed her sister (with two young children in tow) to stay at her place. I was suspicious for weeks, as I had gone about 6 weeks without seeing my tenant's car (I was there daily doing a renovation in another unit) but would quite regularly see her sister coming and going.  My suspicions were elevated when I " a friend of mine" happened to see some mail addressed to the sister being delivered to the unit in question.

I stopped the sister one day and asked her point blank if she was living there now, and she said "no", that she was coming over to "hang out" and check on the place because her sister was staying with her BF.  I didn't mention the mail to her.

I addressed my concerns with the current tenant (whom showed up last week), and she corroborated this story.  I didn't mention the mail to her, either.

 My tenant's lease has a clause forbidding unauthorized occupants for more than 7 consecutive days or 14 total days over the course of the year. 

Here are my questions:

(1)  Is it an urban myth, or is it illegal for me "a friend of mine" to look into a tenant's mailbox as part of an investigation like the one described above?

(2)  Assuming I am correct about the occupant, what the possible consequences for me choosing to ignore this situation? Despite the lease, does she at some point get tenants' rights (I'm thinking something akin to "adverse possession.") or do I have to option to consider her a trespasser if I choose?

At the very least, I am going to give notice to enter and do an inspection in the coming days, just to see what has been going on inside.  

Other facts: current tenant has 2X security deposit down due to unverifiable income, and has been a trouble-free tenant otherwise.  As of this writing, rent has been paid on time since I noticed the shenanigans. (c. early April)

Any tips/advice/feedback is, as always, welcome and appreciated.