[bagster]

Dictionary.com:

Informal. something or someone that becomes unexpectedly successful or important after a period of being unnoticed, ignored, or considered unpromising or a failure: The play was the sleeper of the season.

In the case of Q, a [sleeper] is someone who is a derp but is not recognized as a derp. When Q calls [Wray] and [Coates] sleepers, he means that they were derps that even Trump/Q did not know were derps.

Think Mattis, Priebus, McMaster, Kelly also.

[mbrfl]

I think in the case of McMaster and probably Coats, Q/Q
+ probably knew they were sleepers from the beginning but intentionally played dumb in order to give the DS a false sense of security.