What you are discussing is
holding rather than
resting. 30 minute rest time is leaving the meat on the table (at room temp) to allow it to come down to approximately 145° before you slice into it. Holding, as you're describing, is keeping the meat above 145° for long periods of time to ensure there is no harmful bacterial growth.
If you cook the meat to a perfect tenderness there is no need for holding at all. There's a good article here on AmazingRibs.com about all of these holding / resting myths.
https://amazingribs.com/more-techni...ce-juiciness-why-resting-and-holding-meat-are
@CrotonMark,
The brisket was likely not finished and needed a bit more time. Probing is definitely the way to go and you've got to probe in a few different areas, with the most emphasis being placed on the thickest part of the FLAT. If that area is probe tender then you should be good to go.
A good rule of thumb for brisket is this : when you think it's finished cook it for another 45-60 minutes (if cooking low and slow). Slightly overcooked brisket is much better than slightly undercooked brisket.