I think your two English examples are fundamentally different and do not fall into the same category. He was seen doing something means that someone was observed doing it, while he can be seen doing something means that he does it.
I saw John eating means no more and no less than that the speaker saw John eating. In German, you might say something like Ich sah John essen. Simple as that.
They can often be seen roaming through the mountains, though, doesn't mean that "they" are necessarily often observed roaming, just that they roam often, and could potentially be seen doing it if someone actually looked. To express the same meaning, in German, you might say something like
- Sie gehen häufig in den Bergen wandern
- Sie wandern häufig in den Bergen
- Man findet sie häufig in den Bergen beim Wandern
- Man trifft sie häufig beim Wandern in den Bergen an
- Man sieht sie häufig in den Bergen wandern
or any other phrase that expresses that someone does something regularly.