Many years ago, I was sat in an office by our flight line, when one of our Sea Kings taxied by. I caught a very quick glimpse of it as it passed (a blink of an eye really) and I thought that I saw the front door wasn't closed properly. It was so quick that I wasn't really sure what I saw, so let it go on its way.
I was on the receiving crew when it returned and sure enough the door wasn't latched properly. I had this realisation that if someone had leant on that door, although unlikely, it could have opened and a crew member fall out.
From that moment, if I saw anything I wasn't happy with, I reported it, no matter how silly I could have looked.
Particularly with unfamiliar aircraft, I've seen lots of things that I wasn't sure about and 99.9% of the time it's been nothing. Being made to look silly, trumps an accident or incident any day and you never know, it might just be the 0.1%'s turn.