I would argue that {'op ret} without a unit of time is like saying "at some time in the past" in English, since that's what's provided as the translation for {'op ret} without a unit of time in the qep'a' materials. Was there some other information provided at the qep'a' but not shared online that would suggest that MO meant this to be a shorthand notation for general {'op X} time phrases, and not a phrase in its own right? Based on the way the handout is phrased and presented, it seemed pretty clear to me that {'op ret} and {'op pIq} are acceptable phrases for describing unspecified times in the past or future.

I know that {ret} and {pIq} usually require a time period noun, and if you want to emphasize that the time in the past is measured in, say, weeks ago rather than days ago or centuries ago, I think you should use {'op Hogh ret}. But if you're not specifying a time frame, and just referring to any time in the past, why does it matter whether there's a unit? {'op ret} would just literally mean "some unspecified amount of unspecified time periods ago". Admittedly, this particular expression probably doesn't see much everyday use, given the Klingon cultural reluctance to be imprecise, but I don't see why that would make {'op ret} any more ungrammatical than, say, {jIHeghvIp}.