Hmm, sounds like you were reported for something or other, or perhaps some sort of drone tracking system that can pinpoint the drones info etc, and police were tasked to track you down in the first instance.
Or, you might have uploaded a flight to YouTube or similar that went against one or more of CASAs drone rules ?
Again, it would have to be pretty bad to make someone report you if seen.
Yes, most likely unless you did a flight with a huge or serious risk, you will likely get a warning about a flight or flights you may have done.
CASA are generally still at the educational level for errant drone pilots, but after recreational registration comes in (perhaps in a few months if their schedule is right this time), things might hot up as they will know drone pilots know the rules, no excuses for risks.