Say my opponent's Rattata used its "Dangerous Suspicion" attack which moves it to the Bench. On my next turn, could I use Sudowoodo's "Watch and Learn" attack to copy "Dangerous Suspicion"?
Yes, you can. A Pokemon being benched or leaving play does not keep the attack from being copied by Watch and Learn.
Source: BREAKpoint FAQ (2016-01-23), TPCi Rules Team (2016-02-11)

Relevant cards

Note: All applicable cards may not be listed below.
XY9_67