Notes are the second collectible items in the mappack and make their first appearance in coop_omega5, where they're introduced. Each note has a random letter written, when combined into a correct order, the word might be used in the end/last part of the mappack. Most notes don't appear in maps and some notes can be collected after a sudden event. Here's a list how to get them:
(E) coop_omega5 - Go to the below of the road lines and keep heading left.
(O) coop_omega6 - Go to the right side of the 4-way tunnel. Note that collecting it will trigger a sudden appearance of Attackers.
(R) coop_omega7 - Found in a cell with a live prisoner. After eliminating the 3-man fireteam of Sane Military and going forward, a scream will be heard and then you must go back to the cell.
(C) coop_omega10 - After the officer requests Control to shutdown the tesla gate in their way, back to the suicide guard and keep heading foward.