Cardinals safety Tyrann Mathieu signs autographs at Heathrow Airport on Tuesday morning.

LONDON, England – When Patrick Peterson vacationed in London with his wife, Antonique, in 2013, they were as anonymous as any other American couple taking in the sights.

The Cardinals' star cornerback returned Tuesday morning, and this time, he was stopped by a pack of selfie-seeking fans at Heathrow Airport. While it wasn't an overwhelming gathering – there were less than a dozen people -- it was an indicator of the incremental progress being made in the NFL's quest to globalize its market.

"We know it's something new over here, and we know soccer is huge over here," Peterson said. "When we are able to put European eyes on American football, it brings that much more attention to our game. The league has been doing a great job of trying to build our game over the years."

The NFL is still met with sideways glances much of the time throughout Europe. Soccer and rugby are king, and there is much confusion as to why there are so many breaks in the action.

"There are definitely very varying opinions on it," said Nick Edwards of North Yorkshire, England. "There are a lot of people who are very committed to following the NFL and their chosen franchise. … I (also) find that some people don't take you seriously if you go out with a couple of mates to throw the football around, but who really cares about them."

The London International Series has been a success. The games regularly sell out, and the lineup has been expanded to four games for the first time in 2017, including the Cardinals' matchup on Sunday against the Rams – their first foray to England since 1983. The yearly presence is making inroads.

"The popularity of the sport in London seems to be snowballing," Edwards said. "I have gotten high-fives on the streets of our capital just for wearing my Cardinals t-shirt."

In a city as big as London, it's not hard to find fans. It's rarer in small towns, which is why NFL supporters appreciate the virtual world which easily connects them.

"Up here, it's basically an unknown entity," said Mark Sims of Lincoln, England. "In the small town I'm from, I think I am the only fan, which is why it's so great to have found the Birdgang (online community)."

The NFL has a chance to get its hooks in Europe because of its scheduling. Games are only played once a week, and the afternoon start times in America make it possible to tune in to live action in Europe.

"It's much easier to follow NFL games than NBA or NHL games, for example," said Bernhard Bucher, who lives in Vienna, Austria. "In Austria the early games on Sunday start at 7 p.m., which is quite a nice time. The only games which are really hard to watch live are the last game on Sunday, (Thursday Night Football) and (Monday Night Football). For sure it is easier to follow soccer in Europe, but for (football) fans the time is hardly a problem."

Bucher said the NFL is "still something exotic in Europe" but is making gains. There has been talk of adding a team full-time in London by 2022, and while it would have logistical challenges, Mark Waller, the NFL's executive vice-chairman international, believes it is viable.

"The fanbase is big enough and passionate enough that it can support a franchise," Waller told the BBC in 2016. "I felt in 2007 it was always a 15-year journey. I think we're on track to deliver that. I fundamentally believe we will deliver that."

There are about 40,000 people who have bought season tickets to all of the games in London this year, and as the exposure grows, the interest figures to rise. While the NFL isn't ever going to become the most popular form of "football" in Europe, it should continue to attract eyeballs.

"It's definitely a sport that is still niche in comparison to the likes of (soccer) and rugby, but it's definitely growing over here," said Tom Donlan of Staffordshire, England. "So it's becoming less of a novelty. That doesn't stop me from getting moderately excited when I see someone else wearing an NFL jersey out in public, though, even if it is another team's."