English is too fussy about tenses, isn't it? Chinese would use 被 in both cases. I'm beginning to appreciate the simplicity of the Chinese language at times. There is a subtle difference. First: "When questioned..." describes two things (question and answer that were part of one process which occurred in the past. Second: "After being questioned..." describes two different processes (being questioned and being put into prison) that happened one after the other, though both are still in the past. Still confusing? What can I say? English speakers seem to take joy in more tenses than we know what to do with.

Being questioned is the same as being asked. The sentences are arranged differently but there are several different ways to arrange them which would be correct. For example, the second sentence could also say "After being questioned, the thief was put into prison." and it would mean the exact same thing. As for the word being in the second, it indicates a different type of past tense. He was being questioned in the second. In the first he was questioned. Being indicates that it was an event that was ongoing. The first sentence could also start out as "When he was being questioned by..." and the meaning would be the same