Prisca and Aquila were probably in Ephesus at this time (Acts 18:26; 1 Corinthians 16:19). They also lived in Rome, where Paul currently was (Acts 18:2; Romans 16:3). This suggests that Timothy might still be in Ephesus (see 1 Timothy 1:3).
Paul is grateful when mentioning Onesiphorus in 2 Timothy 1:16 for helping him while he was in prison.