Where must the puma have come from?

    Pumas are large, cat-like animals which are found in America. When reports came into London Zoo that a wild puma had been spotted forty-five miles south of London, they were not taken seriously. However, as the evidence began to accumulate, experts from the Zoo felt obliged to investigate, for the descriptions given by people who claimed to have seen the puma were extraordinarily similar.

    The hunt for the puma began in a small village where a woman picking blackberries saw 'a large cat' only five yards away from her. It immediately ran away when she saw it, and experts confirmed that a puma will not attack a human being unless it is cornered. The search proved difficult, for the puma was often observed at one place in the morning and at another place twenty miles away in the evening. Wherever it went, it left behind it a trail of dead deer and small animals like rabbits. Paw prints were seen in a number of places and puma fur was found clinging to bushes. Several people complained of "cat-like noises' at night and a businessman on a fishing trip saw the puma up a tree. The experts were now fully convinced that the animal was a puma, but where had it come from? As no pumas had been reported missing from any zoo in the country, this one must have been in the possession of a private collector and somehow managed to escape. The hunt went on for several weeks, but the puma was not caught. It is disturbing to think that a dangerous wild animal is still at large in the quiet countryside.

这是新概念3第1课A puma at large, 请教老师几个疑问：

1）表示来源用一般现在时代替一般过去时，例如我俩见面，我问你：Where are you from或Where do you come from, 而不说were, did, 虽然你来自哪里这个动作是过去的但用一般现在时代替过去时。

那么按照这个说法，为什么在课文里出现了Where must the puma have come from? 和 ... but where had it come from? 这2句为什么不用一般现在时呢？

2）experts confirmed that a puma will not attack a human being unless it is cornered. 书上说这里的will不是将来时，是表示倾向的习惯性的用法，对吗？为什么不是unless it was cornered 呢？（注意confirmed是过去时）

3）书上说found clinging to bushes 和 reported missing 这里的clinging和missing都是现在分词，作主语补足语，我没有理解？现在分词就是强调动词动作吧？？我认为这2个都是形容词作主语补足语，对吗？

4）this one must have been in the possession of a private collector 这是站在写这篇文章的时候的时间点说的吧？如果当时在故事现场，是不是应该说成this one must be in the possession of a private collector 呢？