Has anyone visited Mexico or Canada for less than 30 days and?
Yes. This is one of the cases covered by Automatic Revalidation. See 22 CFR 41.112(d)(1)(ii): (1) Provided that the requirements set out in paragraph (d)(2) of this section are fully met, the following provisions apply to nonimmigrant aliens seeking readmission at ports of entry: (ii) In cases where the original nonimmigrant classification of an alien has been changed by DHS to another nonimmigrant classification, the validity of an expired or unexpired nonimmigrant visa may be considered to be automatically extended to the date of application for readmission, and the visa may be converted as necessary to that changed classification. So in the case of your example where someone did Change of Status from B2 to H1b status, they can re-enter the US on their unexpired H1b I-94 after a trip of less than 30 days to Canada or Mexico, and their expired or unexpired B2 visa will basically be treated as if it were an unexpired H1b visa.