Unfortunately a small leak could be a handful of things. Youd be lucky to get a large leak. Typicall it is almosy always a gas cap or some sort of major damage to the evap system.
A purge valve can be the culprit of small and large leaks. There is also a pump in the system whoch can also cause a small leak. Any where air can get past, weird things ive seen that have kicked my butt at work are in tank pump seals leaking out air, cracked charcoal canister, the purge valve installed incorrectly. Alllll sorts of stuff, pretty difficult to diagnose without a smoke machine but still doable with lots of patience.
2005 Silverado 2500hd Duramax
1972 Toyota FJ40
1967 Porsche 912(100% stock)