Is not checking in any baggage suspicious?

I've been to the States three times, flying once from Paris, and twice from London with just one backpack and no checked in luggage. The airports were Newark, San Francisco, and Denver. I wasn't stopped or questioned once.

I live in the UK and until not too long ago, I used to fly between London (LHR) and Boston (BOS) quite often, up to a dozen times a year. Most of these trips to the USA were for short periods, the longest being about a week, but quite a few were literally for a weekend (i.e. flying to USA on Friday evening and flying back to UK on Sunday evening).

For most of these short trips I would only have a 25L backpack with me - with the minimum required stuff - and buying/obtaining anything else in Boston.

Most of the trips (but not all) were with Virgin Atlantic. Not once had I been stopped because I didn't check any bags. Once or twice I was selected for a secondary screening, where they would sift through my backpack - and then let me on the plane.

So I can say that not having any checked in luggage is not a problem in itself.

Note that I am a dual British/American citizen - and this may have had some impact.

Not at all.

I have friends who travelled all over South America and New Zealand with just carry-on 40L packs.

Many low-cost carriers these days also charge for luggage, so for example, when I fly from Sydney to Christchurch tomorrow (international) I am taking only carry-on - it saves me money, and it's fine for me to pack that light for a few days.

Not at all. Tons of people travel with just carry ons. I wouldn't worry about it much, so long as you abide by the dimensions/rules for carry ons.

Based on my experience, the thing that's most likely to trigger extra security checks (at departure rather than arrival) is being a single male travelling on your own.

Every time I've travelled to US in that way, I've been pulled aside for extra security checks at the gate - always very brief and polite. I never experienced that doing similar trips when travelling as a couple or part of a larger group.

I just returned from a 12 day trip to Europe, flying in from Amsterdam to Detroit. I am a single male, 46 years old, U.S. citizen, and I only traveled with a 32.5L backpack as a carry on, and also carrying a small bag. Before I left customs in Detroit I was pulled aside and the entirety of my belongings were searched; they even flipped open the top of my dental floss and peaked in. They left me to pack most of it back up, and thanked me for my cooperation. I had nothing to hide, but still a little unnerving.