I've noticed how the ISTJs that care about me always remember really little details, things that I wouldn't bother to remember at all, like exactly what my likes and dislikes are in terms of practical things. For example, how I like my tea, what films/ books I've liked and why, what restaurant I liked and why.. And then 5 years later they'll bring something up in conversation like "But you said you liked that book because..", and my reply will be "What? I said I liked that? Really, when? :S" Recently an ISTJ who cares about me asks me when my birthday is and I told him, and he repeated it outloud and I could tell it was one of those little details that he was storing up in order to remember it. And I'm sure that he will because he's made up his mind to.
As regards ENFPs, you know, I don't think we use Si to remember things unless it's very necessary. What I use to remember things is really more my intuition..I draw intuitive connections between things in my head. So, for example, if I see a person feeling a certain way then my intuition will make a connection between the way they are feeling and the last time they felt that way, and that way I will remember a past conversation. Not the facts of the conversation though so much as the feelings, and that will enable me to draw out the facts once more. It'll be like "They look tired... that reminds me of when they looked tired before.. when I'm tired I like to drink tea... they like to drink tea too..."Hey sweetie, would you like a cup of tea? How do you like your tea again?" I imagine an ISTJs thought process on the subject would be more like "It's 4pm, they like to drink tea at 4pm because I made a point of remembering that last time, they like tea with milk and 2 sugars. I will make them a cup of tea. Would you like a cup of tea?"
I think that, because we as ENFPs are so bad at remembering the details, we draw intuitive maps everywhere. My way of remembering where I put my keys is "When I come in I feel like this, so my first thought is 'I will put my keys on this table', so that I can crash on my bed because I feel like lying down. So my keys are likely to be on this table. Yep there they are." I use empathy to empathize with my past self and remember where I put my keys. The reason I sometimes get lost with that is "Oh I can't remember how I felt..if only I could remember how I felt I would know exactly where I had put my keys, bother :S".
I imagine that an ISTJ's thought process would be more like "I generally walk in the door and put my keys on this table. It's practical to keep them there. There they are." - using Si to remember what you do.
