Aux fuel tank leaking

[MSGirl]

About a month a ago put 20 gallons of gas in our auxillary fuel tank. The previous owner had never used it, and we had to even cut the factory zip tie off the nozzle. It all worked flawlessly & very handy for filling up generator while boondocking. When got back home pumped the remaining gas out into jug for lawnmower, but I'm sure there was still a little left in the tank.
Fast fwd to this weekend ...chillin at campsite & a got a whiff of gas, was sitting where I noticed a small puddle & slow dripping under tank. Took the gas cap loose, it did have a little pressure, & quit dripping. So thinking it has a vent stopped up & syphoning fuel out the top. I can see it running out between tank & frame rail; nothing leaking at visible fittings, hoses or pump. Would like to drop the tank to look, but because of this artic pkg insulation/mess sprayed everywhere it makes it a PITA to get the fill neck & such loose.
Is there a vent in the top of tank it could be coming out of?
Is this black , plastic canister (in pic) a vent or what does it do?

[Mastertech]

That plastic black box is the charcoal canister . It is supposed to absorb the fuel vapors before it enters the atmosphere.