When is a separate vent not needed?
It could be that you have a single stack vent system:


  The Single Stack Vent System was introduced into the IPC in the 2012 Edition and is based
  on a drainage stack system in the City of Philadelphia (Pennsylvania) Plumbing Code which
  has been used successfully in many multi-story and high-rise buildings for over 100 years.


https://wabo.memberclicks.net/assets/pdfs/Plumbing_Venting_Brochure_2018.pdf (pages 17-19)