mindenf250 is correct, , , , the truck must know it has a trailer hooked up.
HOWEVER, , , , , my 2011 F250 never knew it had a trailer and always blinked the 3 times. I pulled two different trailers with brakes on this truck but it never did the 5 blink. One was a 2008 tow trailer with 4 wheel electric brakes but was never recognized by the truck. The other was a brand new 2012 5th wheel and the t5ruck never saw it either.
NOW, my 2012 F450 has pulled the 5th wheel trailer only. I have pulled this one 6 or 7 times and got the 5 blink about half of the time. What I mean is if the truck recognizes the trailer, it will do the 5 blink every time until I unhook. The next time I hook up, the truck may or may not see it??? I do not know why. This is a huge trailer with 4 wheel 12 inch brakes that are brand new. This trailer uses all 7 connections of the wiring plug as it even has back-up lights so the truck should see it every time it gets connected. If any truck was going to see a trailer, , , it would see this one!!!!
So what I have gathered so far is the 2011+ Super Duties are hit and miss on recognizing trailers. But my F250 never did.