Help!! Shared meter question

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I have a shared meter inspection on a property I just purchased . It's a 4 unit building and I am pretty sure it's wired correctly and I do have a home meter . The only concern I have is it's possible that the tenant who has called for the inspection may have a heat vent that heats a hallway. This hallway is her hallway and she uses it for some storage but it also is the front door to another apartment. I am freaking out a little bit about this so I have some questions . Note: I am completely ok with fixing the issue going forward if it is an issue . I am an honest landlord and try to make everything right and fair . Has anyone been through one of these that can shed some light ? Any idea of possible costs I could incur from 1 vent ? How could I heat this hallway without installing a new furnace ? Has anyone had another finding and can they let me know how much that cost them? From what I read a hallway that is not completely the tenants should be on the landlord so it doesn't look like she has any responsibility to help heat it . Really any information would help me because I can not find much online ! Thanks , Bry