They can't do an RO investigation on every single PR entering Canada, it would take too much time. So they mainly do them on PRs where it's plainly obvious they don't meet the RO by a few simple questions i.e. they have an expired or no PR card, have a valid card but have been away for 3+ years straight, etc.
Those that have a valid PR card and are doing a short trip out of Canada probably have a better chance to enter without being reported since CBSA may not even ask about previous trips outside Canada, but again it's never guaranteed and much comes down to luck.