It seems to me this should be the place where the "lead" shop mechanic and/or a shop mechanic that shows some leadership skills should be talked to in private by someone they respect for their actions and not just their job title, and asked if they notice this is being observed and if they think it is really happening or if this was some sort of special thing that was happening. And if they do notice it (or might notice it in the future, now that it was brought to their attention), ASKING [not -telling-] what can they do to help their co-workers deliver that last full man hour of the day. Maybe pointing out that 6 guys taking 10 minutes each is 1/8 of a man/day as part of that conversation.
I think this is a spot were a 'leadership by example' style can have far better results than a 'management by a big stick' style of employee correction.
Road guys ALSO will follow (just not as close or quickly) someone they respect who feels comfortable talking about how they manage their time to bill and actually work and deliver to the customer 8 hours labor each day, and for road guys, a morning coffee and 'lunch box safety meeting' 10 minute round table discussion seems to me to work best as a place to get safety discussions going , give out important company 'news' and get these sorts of discussions -started- with an expectation that the discussion will continue between the road techs for the next few days.