You can't detect a radar that is in "hold" mode. "Hold" mode allowed a radar operator to stop the signal sent out of the radar. Release the "hold" button and the signal is sent. If done right, a cop can release the hold and track and lock your speed before you can react. That being said, it takes a vigilant, dedicated cop to do that and most cops don't. I rarely do that, but if I see that cars are consistently speeding at 15 MPH or higher over the limit, I might sit and play the game.
The non-cop part of me though says, nice radar and great install. Hope it works out for you. Years ago, I would have been all about getting one of those!