Just recently I've found some props that have a flickering effect to them. The first one I found is a flickering green trash can located in the purple Alyx doll room. The other 2 are the lamp and shelf located in Ratman's hideout (The submarine room that leads to the lake). I've also found 4 flickering globes that appear after a while of playing on the map.

The first globe is located on the 6th balcony up on the right side of the apartment building next to the spawn building. The second globe is located in Ratman's hideout on the opposite end of the room in the right corner. The third globe is located within the room opposite the mirror at the top of the stairs. The 4th globe is located on the roof of the spiral staircase building.

To find these flickering props, look for flickering shadows at the base of the prop.