The best thing to do is be nice when you go in. Tell them the problem and tell them you want to leave it overnight. If you walk in and immediately start a pissing contest then you will probably be ignored or at best placated. A technician has to duplicate or see blatant evidence of a problem to issue a warranty repair. We can get in serious trouble for guessing as the manufacturer calls all parts back for inspection. You can understand why they do this. If this system wasn't in place people would walk in and say my car is using too much gas you need to replace the engine. The OEMs have to pay for that part and the technicians labor (although it is at such a low rate the tech actually looses money). The biggest issue is the OEMs don't pay techs for diagnostic work or if they do it is a fraction of the time it actually takes to figure the problem out. So in this scenario the tech will do a basic inspection and check for codes but since he is NOT getting paid it will stop there unless he can duplicate after 3-5 attempts. If you leave it overnight and he hops in and has this issue he will have a direction to go in and won't feel as if he is loosing money or wasting time.
Patience is your best decision. Be nice and try to identify with your service advisor. It is a proven fact that people will do more for you when they can identify with you. I've been a master tech for 8 1/2 years, I know how this system works.