What does it means on Facebook "messaged X minutes ago" (read details)?

[M]

On the Facebook chat, when I hover mouse over someone's name, it brings up little balloon with picture, name, work information about that person, and in the bottom of that balloon, it says "messaged X minutes ago". It means that I have chatted with that particular person X minutes, hours or days ago.
But it shows the message "messaged X minutes ago" on certain persons that I have NEVER (or at least not in the indicated time) chatted with. It happened a lot of times, with 3-4 persons. What does that mean?
Could it be indication that that certain person clicked on my profile through chat link on my name, or that it opened chat window and stared at it, not knowing whether to write something or no? I observed this for a long time (about 2 months) and I am sure it is not a bug, because I am certain that one of those persons checked my profile, even when we didn't chat.

[YWWE>BVW>mwwe]

it means that is how long its been since they last messaged you