Agree most likely to have been flushed. And most likely a child....unless someone was completely stuck and needed to get rid of something in a hurry. I suppose there is also the possibility that someone had no idea how a septic works (which is possible for someone who has always lived on town water). Most who are used to being on septic are pretty careful what you put down there (ie bleach, some washing powders, wine is a big no no too).
If it was shoved down a stormwater drain it would be unlikely to block it up (being a childs shirt)....and if it did, you would only notice if you had heavy rain and the drain overflowed (or if you emptied your pool/backwashed it I suppose). Stormwater drains have a pretty heavy duty capacity.
There has been no other mention of the shirt to my knowledge and I would guess if a plumber was called to clear the drain he would have run an electric eel down it which essentially would have chewed it up anyway.
