Why is bread highlighted? It's not lit any differently to the other barrels.
This is most likely just a lighting glitch, which for some reason focuses on your bread barrel. For me, all my barrels light up when I point at one with my gravity gun. There's nothing special about it, just a few glitches in the coding. Replace the bread with something else and try a different barrel for it, it should be fixed then.
EDIT: Here, look at some nice pictures of my barrels. The first one shows the lighting glitch resolved, the second one with the light glitches.
HEY! Lookie here! The same problem occurs with signs!