Tenant Did Not Disclose Current Lease

[Account Closed]

I just closed on a two family property and discovered the night after closing that there was a current lease in place. I found this out after entering the tenant’s apartment (which I thought was unoccupied) and activating her security system alarm. I have now unknowingly inherited a lease and a tenant about which I was not given the opportunity to educate myself. I also unknowingly entered my tenant's apartment without giving her any notice or obtaining her consent. 

The tenant was also completely unaware that the property was exchanging ownership.

Has anyone else had this problem? After doing some research, it doesn’t appear that current tenant leases are on the list of things legally required to be disclosed in a seller’s disclosure document. Are current leases legally required to be disclosed to the seller while under contract? How can I avoid such issues going forward?