Modern signal detection and location isn't standing still either. By introducing frequency hopping, you have only added another dimension to an already multidimensional problem (more precicely, you can no longer ignore a dimension that was there, but constant). If you can solve any problem in two dimensions, you can solve it N-dimentionally. It's by no means easy, you do need alot more processing power, and you would need to put it all on a tiny missile, but the way things are progressing.. soon the only sensors you're gonna be fooling are the ones from tha last millenium.
In the end, turning your radar off is the only sure way of it not being detected, much like the only sure way of not getting infected on the Internet is to disconnect. It's up to you how you will manage the risk.