Can travelling alone for a short period of time to the US trigger red flags at the border?

Anything can trigger red flags at the borders but if your paper work is in order, you do not have to fear the interview.

There is no reason to expect problems at the border when traveling from Europe and having a return ticket and a good reason to return home.
One reason they interview young people traveling alone is that those sometimes cross a border to work illegally in the country they travel to. So the officers will ask questions that will show whether that is a real risk with this person traveling.

But the border officers would ask you the same questions if you came into the country from an intercontinental flight or when traveling with a group of people which looks like they have been just joined up for crossing border purposes.

Be prepared, have a copy of your flight itinerary near at hand in your (hand) luggage and tell the truth on all questions, just no more than they ask for.
And unless you expect to break the rules, you will be OK.

No links, as this is a personal experience answer, been across that border traveling alone as a young adult, in a train with a lot of young people and the only one that was interviewed long (and likely denied entry into Canada) had a lot of things that triggered the 'might stay for illegal work' sensors. She was US citizen, not a foreign tourist to the US and Canada.

NO, traveling solo is not in itself suspicious. If it were, I and many here would be on every watch list worldwide. ;)

I would suspect thought that entry for elicit purposes is attempted more often by solo travelers hence a perceptible or non-perceptible level of extra scrutiny.

This concern should in no way impact you travel plans provided you know of no other issues that would make your entry suspicious.