To me, this is a link between the meeting (impromptu or not) and the Agenda category item. The ingrained behavior I think you want to get to is at the beginning of a meeting make a quick mental scan asking "Does this person have their own Agenda in my lists? If so, what's on it?" This is a formalized version of meeting with someone about topic X, and before finishing they say "Let's see...is there anything else I need to talk to you about....no, we're good for now."
When I'm processing an item and deciding where it lives (for now), I may only know "I have to bring this up at the next meeting" or "...the next time I meet with them." I won't go into more detail than that; I may have to go into more detail when addressing it in the meeting. That's the meeting preparation.
I find that the way these Agenda items work best is to accumulate related items through the week (or whatever time period) in the body of the item. Then before the meeting (an appropriate time before) I review everything that's accumulated and incorporate them appropriately into the upcoming agenda (formal or informal).
This can be especially useful for topics you need to address with people who have limited availability, like senior management. If you see them only once a month, you want to make sure address everything you need while you have their attention. You don't want to finish the call/meeting, have them go on to their next meeting, and then remember "oh, wow, I needed to ask them about X...."