If the filter was not in use for 3~6 days... and the moment you plugged it in there was no sign of ammonia or nitrite...
Then this is proof that the vast majority of the bacteria in your system is in the aquarium, not in the canister filter...
Things like this make ya wonder how important bio media is doesn't it?
This is pure assumption. You can not say this for fact at all. Although, bacteria to thrive in the aquarium, even in the tubing after the filter which is weird.

Dont get me wrong I overkill the hell out of everything aquarium related. I am totally one of those guys