My truck is wired so that the trailer auxilary switch energizes the center pin on the plug.

The Meritor Wabco web site has manuals available in pdf format.  Printed page 8/displayed page 12 of my trailer's brake manual has a chart that says "System is powered when ignition is switched on".

I'm not sure what FMCSA rules say regarding having the power on a switch, but not all trailers I pull can have the center pin energized all the time when the ignition is on.

It stands to reason that the ECU will draw power if the center pin is energized.