Extra tenant and dog living in IP unbeknownst to landlord

Last week I went around to my IP to drop something off for the tenants who are a young couple. While I was there I discovered another tenant living there. The couple explained that she had been there for four months and they had notified the PM. There was also a small dog there which may also have been living there. All this was news to me.

What would other forumites do in this situation? Should I raise the matter with the PM? I really don't mind the other tenant and the dog but can I use this to raise the rent? The couple are otherwise great tenants.
In order:

1. Keep banking the rent cheques.
2. Keep drawing down equity when it is available.
3. Keep investing.
4. Don't sweat the small stuff.

Many (OK a few) PI's on here would love to have those tenants of yours :)

Look, if you want, add the tenant to the lease as the rugrat has said. Add some pet clauses to the lease re:
1. Any pet damage they fix
2. When they move they pay for a flea spray if you find any

5. Stop going around there.
I have experienced a similar situation - original tenants (who signed the lease) did not live at property but other people living there. After getting some advice from this forum, I have asked property manager to sign a new lease with the people living there. I could not increase the rent as the new lease is until the expiry of the existing lease. I wanted to leave my options open to find new tenants later. The rent is still being paid by the original tenants - it is ontime. I suspected that the original tenants have been sub-leasing my property for higher rent. The property manager said the property is well maintained.