Now that we have search on "reasonable grounds to suspect" the commission of an offence, a lesser threshold than the reasonable belief that had previously been required – and that had been replaced by stealth – it is relevant to consider the definition of reasonable suspicion developed and applied by the Supreme Court of Canada: R v Chehil, 2013 SCC 49 (27 September 2013) and R v MacKenzie, 2013 SCC 50 (27 September 2013).
There is no substitute for reading the judgments, but the essentials, set out in Chehil, are:
- The central question is: Is the totality of the circumstances, including the specific characteristics of the suspect, the contextual factors, and the offence suspected, sufficient to reach the threshold of reasonable suspicion? 
- "Suspicion" is an expectation that the targeted individual is possibly engaged in some criminal activity. A "reasonable" suspicion means something more than a mere suspicion and something less than a belief based upon reasonable and probable grounds. [26, citing Binnie J in R v Kang-Brown, a case discussed here on 28 April 2008]
- Reasonable suspicion is a lower standard than reasonable belief, as it engages the reasonable possibility, rather than probability, of crime. As a result, when applying the reasonable suspicion standard, reviewing judges must be cautious not to conflate it with the more demanding reasonable and probable grounds standard. 
- The reasonable suspicion standard addresses the possibility of uncovering criminality, and not a probability of doing so. 
- The constellation of facts must be based in the evidence, tied to the individual, and capable of supporting a logical inference of criminal behaviour. If the link between the constellation and criminality cannot be established by way of a logical inference, the Crown must lead evidence to connect the circumstances to criminality. This evidence may be empirical or statistical, or it may be based upon the investigating officer's training and experience. 
- An officer's training and experience may provide an objective experiential, as opposed to empirical, basis for grounding reasonable suspicion. However, this is not to say that hunches or intuition grounded in an officer's experience will suffice, or that deference is owed to a police officer's view of the circumstances based on his training or experience in the field. A police officer's educated guess must not supplant the rigorous and independent scrutiny demanded by the reasonable suspicion standard.
- While a trial judge is owed deference in relation to his factual findings, whether those factual findings support reasonable suspicion is a question of law, and as such is reviewable on the correctness standard. 
- In assessing whether a case for reasonable suspicion has been made out, the analysis of objective reasonableness should be conducted through the lens of a reasonable person standing in the shoes of the police officer. 
- The hallmark of reasonable suspicion, as distinguished from mere suspicion, is that "a sincerely held subjective belief is insufficient" to support the former (Kang-Brown, at para. 75, per Binnie J., citing P. Sankoff and S. Perrault, "Suspicious Searches: What's so Reasonable About Them?" (1999), 24 C.R. (5th) 123, at p. 125). Rather, as Karakatsanis J. observes in Chehil , reasonable suspicion must be grounded in "objectively discernible facts, which can then be subjected to independent judicial scrutiny". 
- Exculpatory, common, neutral, or equivocal information should not be discarded when assessing a constellation of factors. However, the test for reasonable suspicion will not be stymied when the factors which give rise to it are supportive of an innocent explanation. We are looking here at possibilities, not probabilities. Are the facts objectively indicative of the possibility of criminal behaviour in light of the totality of the circumstances? If so, the objective component of the test will have been met. If not, the inquiry is at an end. 
Chehil also discusses the use of profiles , rejecting it as a basis for suspicion. Here the judicial reasoning is perhaps politically correct, if too subtle, if profiles are based on experience.
MacKenzie mentions facts such as nervousness, red eyes, and erratic driving. The Court split 5-4 on the facts in this case, perhaps illustrating how, notwithstanding the utmost care in being clear about the criterion of reasonable suspicion, its application to particular facts can be controversial.
The difficulties are apparent from this extract from my Misuse of Drugs text, para 1406(a) (citing cases not available online):
But assertion of rights is not properly a basis for reasonable suspicion, as was stated in Chehil at :