The Blues were among several top clubs represented at the Stadio Georgios Karaiskaki as Sporting got their Champions League Group D campaign off to a flying start against Olympiacos.
The Portuguese outfit were 3-0 up by half-time with Martins, 22, scoring their second goal as he broke away using his pace and slotted past the Greek keeper.
Martins - who Chelsea have tracked for over a year - has been in good form this season with three goals in five league games for the Lisbon club.
The £50million-rated wideman has previously caught the eye of Real Madrid and Liverpool, but neither of those clubs had scouts in attendance on Tuesday.
Bayern Munich, AC Milan, Villarreal, Eintracht Frankfurt and Juventus, who are also in Group D, were the other clubs to send their spies to Piraeus.