huh?

It probably shows if
a) it's damaged, and
b) you're within a certain range of it or
c) you hit something in the map.

If the boss has been damaged, then the boss bar will pop up as soon as you get near it.

This is correct.

It's not another glitch, people.

I thought that it showed up when either the boss's hp or mp changed when you're in its range. Bigfoot might recover hp or mp or something.

Well then, mystery solved. Still seems like a dumb reason for the bar to appear.