Fernando Alonso spent Tuesday at the Circuit de Catalunya.
The arrival of the former F1 driver at the final winter test followed speculation he might even drive the 2019 McLaren.
He was wearing McLaren team gear, and according to La Gazzetta dello Sport, Alonso even has his helmet in his luggage.
Alonso was accompanied by his manager Luis Garcia Abad, and amid ongoing rumours that he might mount a 2020 return to F1, he even spent time with his former Ferrari team.
He said: "My focus is on preparing for the Indy 500. Next week I’m flying to the US to work on a simulator.
"I spent 18 years in formula one, and for now I just want to enjoy time in the paddock," the 37-year-old added.
McLaren team boss Zak Brown would not shed light on whether Alonso will be testing, but sources indicated that there are no such plans.
"Fernando just can’t stay at home anymore," Brown smiled, according to El Confidencial. "He’s going from place to place and saw the cars on the track in the afternoon."
Alonso said he is willing to help McLaren.
"We were in the meeting room talking about how to solve the problems of the tyres. I told them they need to understand the problem in five laps," he joked.