4 plex with a smelly public area

[Richard Porras]

I am a landlord that lives in unit 4 of his 4 plex and rents out the other 3units. About 2-3 days ago I noticed a smell in the common area(hallway) when I walk by unit 2. This person has not been around for 2-3 months because her dad past away, but the rent keeps showing up (when I ask). I am not sure but I think the smell is coming from unit 2. How should I go about addressing this issue?