Hi Ian
Was this on Vatsim mate?
I often just turn up at an airport, usually sitting in a Cessna somewhere and tune to the tower or other atc to listen. I don't normally file a plan, and have found I get left alone till I file a plan or atc is bored and wants to chat lol. They normally text you when you need to contact them.
I don't see a problem, but you mentioned you contacted tower.
I would suggest that you could file a flight plan if you wanted, even to say 'parked at gate until xxx time', or 'observing', but you don't actually need to contact tower or atc unless you are going to be occupying air space.
Perhaps it was a busy time, and as you weren't going to go anywhere didn't need to contact him, so he wondered what your intentions were? Remember that being asked your intentions is usual practise, although it can seem confrontational it's not - they just want to know what you're up to. I'm sure Alex or Chris D will back me up here.
Maybe it was a bad time, it seems like a stressful job, I am often amazed at how calm Alex comes across on the airwaves, even when faced with a pilot using the callsign ZK-YARRRRR
Anyway, don't take it to heart.