If it's in the main pipe, the only place it could have come from would be the breather. Probably running through into the throttle housing too. If it's running fine, I'd be inclined to leave well alone.
Ticking the engine over won't shift much of anything - it needs a good blow out.
As for coming on at fill up, unless you put the nozzle in a strange orifice, no.
More likely just turning the ignition on and off. I'd still suspect a lambda sensor. One member here found that simply moving the lead from it, corrected the fault - slightly dodgy bit of wire and iffy contact.