I've recently got back to writing more of my SF novel (after a lengthy interval) and picking up the threads again, I find myself instantly confounded by a problem that has been waiting for me all this time: detecting spacecraft at a distance - by which I mean distances counted in dozens of AU. One given in terms of communication is the preferred use of lasers rather than radio. That would make detection by third parties fiendishly difficult, I should imagine. I guess too that trying to spot a spacecraft's fusion exhaust at such colossal distances is also one for the tooth fairy. All this being so, would it be theoretically possible to use radar as a means of detection? I have no objections to pushing the boat out a bit in terms of future technology, but I don't want to transgress the known laws of physics if I can possibly help it. Any tips or helpful advice gratefully received.