Since it's your third purchase, I doubt they want to lose a deal over a "$500 loyalty discount"...
I would approach it as, "I think you forgot to include this" rather than accusing them of an intention to screw you.
But that's me. In negotiation, I've found that being pleasant goes a long way.
Regardless of what deal you make, you could always make one better.
Carried to its logical conclusion, what is an endless search worth?
At 73, time is probably your most precious commodity, or it should be.
Why waste it at a dealership if you already have a deal that you are happy with?
A good friend of mine had a quote he used often, and seems to be applicable here.
"Don't let the perfect be the enemy of the good".