Let's say I'm with some people, and they show me something that once I knew. Would it be correct to say:
Ah! I had forgotten about that!
My concern is whether the past perfect can be used or not, since there's no time reference about an action being in the past of a past action.
It sounds good to me, and I guess it could be correct because it's implied something like:
Before you showed me (...) that!
But, if you have just showed me (I'm literally saying this right after), I don't think that using the past-simple is ok, so that would mean that the past-perfect is uncorrect.