A person might need to sneak back into an apartment to retrieve belongings. But he shouldn't need to do so for months on end. And there would not be "lots and lots of people." The lawn issue is secondary; you may have, quite literally, a den of thieves next door to your long-suffering tenants. I think you ought to make one attempt to contact the landlord (this information is easily available through the New Haven Grand List, office of the assessor), and if he does not come up with a satisfactory answer, then contact the Livable Cities Initiative.