I tend to notice vehicles following me for an inordinately long time (but that’s because I’m ex MI5 and an ex Ninja 
), if it’s another lorry I barely give it a thought, if it’s a car I just assume that it’s someone using their phone illegally and using me as a shield. A van (as mentioned earlier) would pique my interest a tad more but as the tat I carry is probably less valuable than the fuel in my tank I’d not get overly worried about it. All in all if you’re benefiting even a little from my slipstream then fair play to you.