The youngster attracted further attention during Estoril's 4-0 defeat against Porto, when he made his first senior start in the Portuguese top flight.
Although the result was disappointing for his team, the match provided an opportunity for several clubs to assess a player whose versatility is becoming an increasingly attractive part of his profile.
Barcelona were particularly interested, with a representative reportedly attending the match at the Estádio do Dragão specifically to watch Arnolin.
According to Portuguese newspaper Record, the Catalan club have followed his development since his appearances for Portugal's Under-18 side. His subsequent progression to the Under-19s has done little to reduce that interest.
Arnolin is naturally a centre-back, but his ability to operate in defensive midfield was another aspect of his performance that caught the attention of scouts.
Against Porto, he was deployed in a holding midfield role rather than his usual position in defence. That adaptability is understood to be one of the qualities Barcelona value as they continue to assess young defensive talent.
The teenager's first start came in difficult circumstances, with Estoril eventually suffering a heavy defeat against one of Portugal's leading clubs.
However, the presence of scouts from three different countries underlines the growing interest in his development.
Brighton have also been monitoring Arnolin, while French side Lille were represented at the Porto match. The three clubs are understood to have been following his progress, although there has been no indication of a formal offer or negotiations at this stage.