The High Court in Glasgow was told Qounjai Channarong, 43, was at an HIV conference with friend and colleague Khanikporn Satjawat on November 11 and 12, 2012.
Miss Channarong claimed her colleague had complained over dinner that a security pass with her name on it had been checked a lot.
She said that when they went to a lecture the following afternoon Miss Satjawat went to the toilet and after around 20 minutes when she hadn't returned, she started to worry.
The jury heard Miss Channarong tried to contact her friend and looked for her and later found out she was dead.
Miss Channarong was giving evidence at the trial of Clive Carter, 35, who is accused of murdering Miss Satjawat by repeatedly striking her on the head and body with a fire extinguisher at the Clyde Auditorium, on November 12, last year.
He has pleaded guilty to the culpable homicide of Miss Satjawat and has lodged a special defence of diminished responsibility, but the Crown did not accept this plea.
Carter, from Motherwell, is also accused of committing a breach of the peace at the hotel on by entering a woman's room uninvited with a fire extinguisher and scaring her.
The court was told Miss Channarong and Miss Satjawat - who work for Bristol-Myers Squibb as product managers - arrived in Glasgow from Thailand, as part of a group of delegates.
Advocate depute John Scullion asked: "Do you know what time it was Khanikporn Satjawat left to go to the toilet?" The witness said it was "almost 2pm".
He asked: "Did Khanikporn Satjawat return to the auditorium after going to the toilet?" Miss Channarong said: "No."
The trial continues.