When for instance an AES-key is being used by OpenSSL, it is put into a 'struct aes_key_st' which is not random at all but quite easily recognizable when scanning memory.

The Cold Boot attack paper by Halderman, Schoen et al. here

https://citp.princeton.edu/research/memory/

...discusses this in detail in chapter 6, Identifying Keys in Memory.

EDIT: fixed the reference