Tomb Raider I-III Remastered Starring Lara Croft

Tomb Raider Unfinished Business - Temple of the Cat (Level 4)

You will collect two keys you can use at the upper right door outside the temple. Head inside to find an alleyway that's not very dark on remastered graphics, but is on original graphics. Make a save and head down the hall to the left, on the right side of the alcove is a switch to turn the lights on. If you want secret 2, reload because this switch locks the door down a further hallway.

Video:

This achievement is earned in "Temple of the Cat".

Once you collected the 2 first keys and open the door from the outside, turn left and look at the wall for a lever.

In the remastered version, it won't make any difference, but if you set the screen on the original graphic, you'll notice a lot of darkened spots in the alley.

This lever will light up the entire path, BUT will also close the door to the 2nd secret, so explore the alley first before using it.