Canada

“Rapporteur”?

Gee. Tough luck! A foreigner stopped at the border and questioned (not interrogated!) about the purpose of his visit. That's what the CBSA is supposed to d. It was probably the first time they ever heard about the position of "UN special rapporteur", and discovering he is a _former _ UN employee, privately investigating human rights violations raised some red flags. Anyway, the CBSA likely had no idea what to do about this man, so they kick it upstairs until it finally gets to Foreign Affairs (or whatever its called by this government). That takes some time. As the man said himself, the questions were not aggressive.
It is entirely normal for governments to inquire about the purpose of a visit. Even I have been asked. But I never said "to investigate human rights violations, like I used to do for the UN"! It would be irresponsible for any border agent to dismiss the issue.
He believes his delay (detainment?) is part of a "global push" (conspiracy?) to punish truth tellers. Full of oneself much?
Also, even a Canadian Senator needs an appropriate security clearance and a "need to know" before getting answers about why.
Finally the "Palestinian Tribunal" is merely a meeting of interested parties hoping to influence the Government of Canada's policy towards the violence in Gaza. This whole incident is hardly newsworthy, but merely Canada protecting citizens and other visitors from harm.
So, yeah, tough luck.