For those who don't have a Q or Z code cheat sheet;
ZKR = I am (or ... is) maintaining watch on... kHz (or MHz).
ZKR? = On what frequencies are you (or ...) maintaining watch?
The following was a common bcast from CanForce via S4285 mode but apparently is no longer sent in the clear, wondered where they'd gone to;
"NAWS DE CFH ZKR F1 2822 3394 4158 6242 8324 12371 16552 AR" 11.14.16
The upshot of all this is one might expect to find voice, ALE, S4285, or perhaps even FSK from Canadian afloat units on the freqs given as the shore stations maintain a listening watch. The shore response to a call may be on an otherwise idle/looped S4285 signal.