You stumbled upon the old remains of seems to be an underground facility after wandering around a sort of office space there are some lockers open and you see a keycard in it 'Level 2' it says, you take it and keep exploring you soon come across what seems to be a checkpoint, so you use the keycard to open it and go into another part of the facility, you go in.

You hear a deep,distorted laugh behind you and you turn to see a tall person that looks decomposed, you start running as he follows