Officially on the exposures page, the provinces asks:
"If you have visited one of these sites on the dates/times (below), you do not need to self-isolate. However, you should self-monitor for symptoms and seek testing if symptoms develop"
When the province puts out a notice, it's supposed be in cases where they arent confident they can privately sort out and get in touch with potential close contacts of a case. In an office environment, you get a list of co-workers and get in touch. Generally speaking a place more frequented by the public like a grocery store would I'd think be a good example of a situation where you can't possibly get in touch with everyone who was there on that day. Someone can add if they can more specifically cite, but believe they also factor in stuff like whether the case was symptomatic at the time and nature of any contact with the public.
Will just point out that we havent seen either grocery store case or the recent Qdoba staff one show up on the provincial exposures page. Not clear if that's because they're still doing the contact tracing on these cases, behind on updating the list or if there's been a determination on likelihood of exposure that led to not being listed. Either way, at this point with case numbers on the rise (+16 in Brandon in the update today, +56 over the past seven days) and with exposures popping up in busier places like this I'd go on the assumption that we're seeing more community transmission potential than we've seen in some time and that it could literally be at anywhere at any point in time. Puts extra importance on all of those fundamentals they've been preaching for a year and extra emphasis on the idea of staying home unless needing to go out, making use of curbside pickup where possible and all of those things that keep contacts down. If you have symptoms, get tested ASAP, stay home and follow directions given when tested