The Sr. Engineer should definitely give (presumably another Sr. Eng.) Bob's code a quicky review and approve it. If Mary or Rando are Sr. then they should get the same level as well. If anyone is a Jr. they should get a much more in-depth review as it's a teaching opportunity, whereas Sr. on Sr. reviews are done to enforce conventions and to be sure the PR has an audience (people take more care when they know other people will look at it).