The reason Comey didn't ask Trump what he meant by "that thing" is obvious:

[Amaryllis]

Comey knows the story of Brer Rabbit and the Tar Baby, and Comey is too smart to stick his fist into the Tar Baby. That was very clear from his testimony. Early on, he assessed Trump well enough to know not to engage with him any more than absolutely necessary, because anything he said could easily become a no-win, or metaphorically, a fist in the Tar Baby.

Yet in the hearing, the Rs kept asking Comey why he didn't ask T what he meant by that. Rs: Checkers, Comey: chess.

[janx]

1. Remind me--

was this during a phone conversation?

[Amaryllis]

2. Don't remember; Trump referred to "that thing" at one point- it was in Comey's testimony, and the Rs

kept asking Comey this morning why he didn't ask Trump what he meant by it, implying he was remiss in his duty. Comey had made it obvious why he was trying to engage with T as little as possible.

I think it was in Comey's statement that was released yesterday if you want to find it.

[GeorgeGist]

3. Trump hadn't forgotten Comey's 'support'.

The full text of FBI letter announcing new Clinton email probe.
https://www.usatoday.com/story/news/politics/onpolitics/2016/10/28/comey-letter-text/92907470/

[ecstatic]

4. He didn't ask for more details because he said

It appeared Trump realized mid sentence that Comey never agreed to anything, so Trump improvised on the fly and said "that thing."

When people start to blurt out something that they realize they shouldn't say, they may either stop completely mid sentence or suddenly switch to cryptic language to limit the damage of what they (accidentally) blurted out. Trump is known for the latter because he's mostly ignorant with a poor attention span.

Example: Person A is about to ask Person B how many months pregnant she is. Person C is behind person B and frantically (but quietly) gestures to Person A "no no no no!"

Person A suddenly realizes he's about to ask an offensive question. He's not a quick thinker, so the question ends up being, "How many months until that thing?"

Awkward silence. Person B doesn't ask for clarification.