None of the Half Life series has stealth mechanics as such. I just don't want to waste my efforts going all ninja if the enemy will be able to detect me, anyway. Yes, I realize I have too many questions.

If they detected you, will they be permanently be on alert for you, or will they go back to their previous routine once you have been out of their sight for some time) Can you can sneak up on them if their backs are turned to you)?

I would like to know what stealth elements are in Black Mesa and how does the enemy AI detect the player.