I was re-reading the manga today because nothing other happend ^^".
So I noticed something, when Ichigo and Renji have there fight in the real world, and Renji is almost at it to make the final strike and then Rukia attacks Renji so that Ichigo could get away, Ichigo goes to the secound time total out [1st time by the meno] and gets a littel insane.
Maybe after he first goes full out [fight with the meno] the hollow in him was created, because the second time he goes total out he gets (a littel) insane and also has an equal laught to Wichigo [chp 55 page 60].
[EDIT:] Renji also talkes about a "abnormal behavior" about Ichigo, so maybe this could would be the first time we actualy saw the hollow
Just a littel theory...
Sorry for my very bad english, but english is not spoken in my country
I didn't find a thread, If one exist, sorry my fault, kill the thread