Aside from XP and cash, bonus crates are a good source of items and, if you are lucky, glu gold. If you happen to die in a mission, you lose any XP, cash and glu gold from these crates. However, items are a different story. You get to keep them. Thus, this is one good way of collecting items for free.
Bonus crates are not required to finish the mission. There are only three bonus crates in a mission so once you already got the three crates, you can complete the objectives and head out for the exit. You usually get XP and cash from these crates and, in a good mission, you may get 1-2 items.But be warned. Picking up a bonus crate would also signal incoming zombies.
These bonus crates are randomly scattered all over the place. Sometimes they are clearly visible; sometimes they are hidden really well. Here are some unlikely places where you may find the bonus crates.
Tents - Sometimes the crate is just in the entrance of the tent.
Behind the wooden crates - These are the ones in the underground lab missions. Just quickly enter the room with the crates and check behind them. Then again, if you are after collecting all the bonus crates, I do advise checking all the rooms.
Corners of storage rooms - Sometimes you need to change the camera view just to see the elusive crate.
Dark alleys - In outdoor missions, there should be about 5-6 alleys. There is a likely chance that one of the bonus crates are in there. Expect zombies lurking there as well.
Near the gas station - The crate can just be hidden behind the car or somewhere nearby.
Inside the vans - Sometimes you miss it because it is hidden by the van door.
Near the helicopter wreckage - Usually to get the crate, you need to go around the wreckage.
Comments and feedback welcome.
The Wolf Pack are my friends, like all others whom I have the pleasure to know. You have my respect because you deserve it - the same way I expect this respect from you as well.