The EVAP service port is located in the EVAP purge pipe between the EVAP purge solenoid and the EVAP canister. The service port is identified by a green-colored cap.
Check the max pressure which should be listed on or near the port. I believe it is 1 PSI.
As far as what it could be, it could be anything. When you have a small leak, the best thing to do is what you are doing. You could have a cracked vacuum hose, something dry-rotted, and so on. I've even seen a small amount of rust on the fuel filler where the gas cap mounts cause it.
Try the smoke test and let me know what you find. Also, in the attached pic, I highlighted the service port.
Let me know if this helps or if you have other questions.
See pic below.
Thursday, April 15th, 2021 AT 1:50 PM