Does anyone know how the monster spawns in this game works, exactly?

I bought this game on GOG (great release, BTW) and I'm planning on playing this again very soon, but this question is intriguing me. I beat this game a while ago and I remember some interesting things about their behaviour - I know there are some monsters that you must trigger their spawn (such as the brute that breaks those bars in the sewers and that grunt that shows up when you get the key on the wall), but is it a general rule in this game or is there an element of randomness involved? I mean, do monsters only spawn when Daniel steps in a certain area, despawn when they reach another area and never appear again?

I'm asking this because I always try to understand as much as I can of a game when I play it the second time around, and this is certainly a mechanic worth understanding. Having monsters that appear only in a certain area and despawning forever means I can explore everything without any kind of urgency involved after hiding from them once, and it would most likely kill a great part of what made this game fun for me in the first playthrough.

Some are triggered, as you said. The others spawn when you enter an area, but it's random(ly determined by you) when you'll run into them. In the latter case, it'll seem like the grunts randomly appear constantly, but really they just lose track of you for a while until you run into them again. Only triggered monsters (and only a handful of those) will truly despawn after a certain time.