The posts by scylla cover this scenario.
For emphasis: impossible to predict how this will impact the decision made.
Unless there are some other chinks/cracks in the case, I would expect this to NOT be a problem, but it is possible it could result in further scrutiny. Probably prudent to wait and see.
A longer response with some elaboration and clarification:
While I agree with scylla that to some extent there is an element of luck, such as in who the Citizenship Officer is making the decision, I think it has a lot more to do with context, with the officer's evaluation of the information relative to all the rest of the applicant's circumstances, history, and so on.
For example, the scenario reported by keesio is one in which it would be very easy for the officer to readily conclude the return to Canada was a day-trip.
In contrast, if there were other weaknesses (chinks/cracks) in the case, in conjunction with those an officer may proceed to make further inquiry.
There are way, way too many variables to sort through and attempt an objective assessment of how a particular CIC officer will consider this.
Many have reported, similar to scylla's partner, that this situation has triggered RQ. Some have reported negative decisions by the Citizenship Judge based largely on unreported day trips (including applicants whose RQ was an earlier version not requiring declaration of day trips, just as the Residency Calculator for applying did not prior to June 11), although the latter has happened in cases with other obvious issues (relatively recent one was an applicant who left Canada after applying to work in the U.S.).
Most, however, do not encounter a problem. But then, for most it is not even asked about at the interview. (I brought up day trips at my interview, because the interviewer asked if I "declared all trips" in my residency calculation, and I answered there were some day trips which were not declared).
All you can do is wait to see what happens. You could apply for your border records from the U.S., but most reports indicate the response time for these is still sporadic (mine took a lot longer than the entire timeline for processing my citizenship application . . . I was anticipating RQ, so requested the U.S. records shortly before I applied for citizenship . . . so they would be current to almost the date I was applying . . . but did not receive the U.S. records until many months after I had already taken the oath . . . as it went, I did not get RQ'd . . . I had several dozen trips to the U.S. including nearly two dozen day trips, no stamps for any of those trips).
CIT 0520 rather than RQ:
While RQ was common in the past when the CIC officer had questions related to whether or not a particular entry was related to a day-trip or not, these days it is more likely (it seems to me) that you would be sent the CIT 0520 request asking for specific documents rather than a full blown RQ. Of course this depends on the extent to which the reviewing officer has questions, which in turn depends, again, on all the facts and circumstances and applicant's history.
It is possible to receive a request for the U.S. records of entry into the U.S. However, I think CIC can now access these directly . . . but it is not clear that this is so for older crossings, or otherwise how far back they can do this. Hopefully they can, and if there are questions they will, rather than request you to obtain and provide them, since again obtaining these records timely can be difficult.
Prudent to wait and see.
Examples of positive factors:
I bring this up more to reinforce the observation that most applicants will have no problems relative to this or that day trip.
What CIC might be concerned about would be that the return to Canada reflects an unreported trip, and more significantly a trip of unknown duration.
The first, the fact of an unreported trip (if CIC infers this is in question) is relative to credibility itself. Failure to declare a single brief trip (two or three days or such) probably does not cause much of a concern, does not raise credibility issues, at least if it is an isolated mistake.
But if and when CIC infers there is reason to think there was an unreported trip, the potential duration of that trip may loom large. This is where the applicant with a lot of additional indicators of particular dates of presence in Canada is at an advantage. For keesio for example, if CIC had reason to suspect a particular date of return to Canada was related to an undeclared trip, the fact of frequent previous entries likely reduces the potential duration of any such trip. In particular, for example, if there was a separate reported exit and entry within a couple weeks prior to the suspicious date, that would definitively limit the duration of the allegedly undeclared trip (say a declared trip ended with return to Canada August 3 and then there is an entry for an undeclared trip that CIC is suspicious about on August 17 . . . even if CIC concluded there was an unreported trip, its duration could be no longer than August 4 to August 17).
Many believe frequent travel leads to more suspicion. I think this overlooks how important context is. It is well apparent that for some applicants frequent travel actually bolsters their case: all confirmed dates of re-entry into Canada show, after all, positive dates of presence in Canada. For the NEXUS holder, for example, if the pattern of travel corroborates the lifestyle of someone primarily living in Canada and making frequent brief trips to the U.S., that actually tends to bolster the credibility of a residency calculation which is consistent with what is discerned from the CBSA travel history, and overall positively indicate the individual is resident in Canada.
There are many examples. The applicant with employment that is readily verifiable and which is the kind of employment requiring the applicant to physically work at a particular facility or place of employment in Canada, is similarly in a strong position despite CIC entertaining questions about a single date of return to Canada. In contrast, for someone like me, self-employed and doing work using telecommunications, work capable of being done virtually anywhere (anywhere there is high speed internet access anyway), if CIC identified a questionable date of return for me, it is likely that would have been more problematic for me.
Again, I go into these for the purpose of highlighting the potential range of variables; way, way too many to do anything other than indicate vague, inconclusive generalizations about what might tip CIC to go in this or that direction.
