They not only can, if they look, see a lot more than what many might anticipate, they are a lot, lot more sophisticated these days in discerning inconsistencies in the information provided by travelers.
And at minimum, the PR's last date of entry will be readily accessible to a CBSA officer at the POE, and if that was more than three years ago, none since, the odds of further screening elevate proportionately.
Main thing is that in the customs' declaration the PR is required to declare how long he or she has been outside Canada. Despite how many may have gotten away with fudging this in the past, or being evasive, these days the best approach is to be fully honest and prepared to make one's case the very best one can. Be sure to have in one's hands (carry-on, not checked baggage) any documents supporting one's reasons for not coming to Canada sooner.
Remember, misrepresentation made for the purpose of obtaining entry into Canada is not only a stand alone grounds for revocation of PR status, but grounds for exclusion from Canada (I think the duration of exclusion has increased to five years), and possibly even criminal prosecution.