How to Prove Tenant on Lease no Longer Occupies Unit

[Ash P.]

We are convinced a tenant no longer lives in his unit, but sublets it or uses it as an Airbnb. We've never seen him on sight, and when we knock on the door (maintenance, etc.) he's never there. He pays rent on time and responds to emails, etc. 

If he is subleasing, Airbnbing, or not occupying the unit he's in breach of his Lease and should be evicted. Anyone deal with this, or have any input? Thank you!