Keeping them on the primitive scale you picture could work with no known evidence of their having "achieved" that.
Consider all the things above about physical evidence, then consider that we might not even realize evidence existed. That doesn't apply to troodons, I suppose, but consider anything a fair deal larger would have houses, for example, on a much different scale than humans. Posthole remnants, often all that remains, would look more like circles in very old henges, if one even realized they went together because of the out-of-the-box scale. Like the recent enlargement of the scale of the Stonehenge site. Eventually realized, but... "eventually"... and archeologists are like anyone else, not wanting to have everything they do ridiculed because they dealt seriously with a workers' "campfire story" conflation (?... or was it?) of odd pieces of evidence. Could take a LONG time for serious analysis.
But... in the example we have, human history, seven million years or so of differentiation was needed to reach the "a few small cities" stage and there is a lot of evidence along that time path. So that would be there. Just how much would ever be in places we would explore is the question. If it was inside a reasonably new mountain, well, even construction projects might never find it, much less notice it.
(Funnily enough, the ones most likely to interpret any such evidence correctly, the "it was ancient aliens" crowd, is also the least likely to ever really want evidence to interpret.)
And evidence can be misinterpreted too, giving rise to longstanding, unimpeachable beliefs that impede scientific explanation in various ways. What are two (of many) things common to people almost everywhere? One is fossils exposed by weathering, even in mountains, many of fish, thousands of feet above any known sea and far from them all too. Another is flood myths. Find a fossil fish a mile up a mountain and 500 miles from sea and a flooded earth story just makes sense. And might easily be more believable with a god as the agent since no natural event floods anything, much less the whole earth, a mile deep for the fish to get there and die. Another common myth is misshapen monsters. Ever seen some of those fossils folk find? We have common ancestors, the basic bones are all there, the heads misshapen, the proportions wrong... must be monsters and see the size??? Gods and their enemies... Even give them animalistic interpretations and all you see is monstrous animals, not normal, natural ones. It is all so easy. Hence my belief archeology might go a LONG LONG time with evidence they shoehorn into sketchy seeming explanations backed by "I'm a professional archeologist and you're the fellow who balances the IQ scale at 100." Or just shove them into a shoebox and store them to never even film for outsiders to view, much less see or handle. A fairly small-scale four-five town civilization might leave traces that no one ever put together.
Then the comet hits and that's that.
However, all that aside, you cannot take them much further along the scale. Let them reach a fossil fuel burning level and the fossil fuels are GONE. Never to be renewed as the activity that created them cannot happen on any meaningful scale ever again. We WOULD have noticed that and if enough were left, we would have advanced to the point where we realized a great deal had been used up. Might not have to go far along the path either because coal miners would surely have discovered enough evidence to get people thinking. It simply couldn't be missed for long, and would have been discovered long before we got going discovering our own past. (Might even have kickstarted that archeology, actually.)
And we would have never reached our current place in the sun. Wouldn't worry about what came before, just curse it.
By the way, it's not a pretty future for our own species if we peak and fall back. Kinda hard to go from campfires to nuclear reactors in one step. Even windmills like today's take a lot of tech advancement.
So a fair ways back on the physical advancement path could probably slide under the radar pretty easily. But much more and it would start to be difficult to picture. Those fossil skeletons we find, they made it. One thinks a stone or concrete building or two or ten thousand could survive with noticeable over just as long a period would not be hard to picture either.