The question flows from the assumption that Thomas was the ONLY disciple who was absent during the first apparition . What Jn 20:19 says is:
Then the same day at evening, being the first day of the week, when the doors were shut where the disciples were assembled for fear of the Jews, came Jesus and stood in the midst, and saith unto them, Peace be unto you.
The term 'disciple' applies to both the Apostles as well as to the 72, the group that may have included women disciples. Of course, the Apostles exclusively met in important meetings.But on this day, they were meeting with the only agenda of saving themselves from the Jews! Luke 14:24 mentions two disciples, one being Cleopas,travelling to Emmavus seven kms away from Jerusalem when Jesus appeared to them. The unnamed disciple could very well be an Apostle.
Mind that only Mary Magdalene had seen the resurrected Lord whom she first mistook for the gardner. Peter and John relied on circumstantial We evidence . We do not know how fast the news of the Resurrection reached , and was taken for real by all the 11 Apostles, some of whom may have been hiding in isolation, so as to enable them sum upsummon courage and get together on Sunday evening. It is therefore, quite possible that apart from Thomas, a couple of other Apostles were absent during the first apparition. Thomas stands out for the very reason that he straightaway refused to believe . The other Apostle(s) who was(were) absent might have readily believed in the resurrection.