Exact same thing happened to me last year!
Got pulled over for no front plate, cop gets out of car, asks for my license, tells me why he pulled me over & then strolls on back to his squad looking all the time at the car.
About 5 minutes later comes back & hands me my license, tells me not to worry, he's not going to fine me for the plate & then asks me "what kinda car is this?"
Sooooo..........did he pull me over for the plate & just to satisfy his curiosity??