You have raised quite a bit in that hypothetical Sidewayz and, like is so often in the law, there’s no black or white answer. But no, generally police don’t need a warrant to search yr car if they have ‘reasonable suspicion’ of a range of things eg that you have drugs, weapons, you have just committed a serious offence or about to etc.
What constitutes ‘reasonable suspicion’ is sort of complicated and any answer could go on for ages. For example I read a case a while back where police found drugs in a car and they admitted that they searched the car because it was driven by a young male, it was a ‘hotted up’ car that looked like it had been damaged at some stage and a rego check revealed it didn’t belong to the driver (it was a NSW case from around 2001 - I could look it up if your really wanted). They found drugs, but the driver argued it was an illegal search and he won in court, the magistrate finding that wasn’t sufficient ‘suspicion’ to do a search. I don’t have enough first-hand knowledge of cases of that kind to know how often police claim things like you have suggested e.g. “ I smelt weed”, or other examples eg “driver appeared drug affected”. However if they claimed something like that, and they found drugs, my guess is that you might be in for a tough time arguing it wasn’t a valid search. Fortehlzuz could probably tell you more, but he doesn’t seem to have been around much recently.
So as you can see there may be some repercussions if they don’t have grounds for a search and they find something, depending on why they say they did it. If they didn’t find anything there would also be some options, but unless the search caused you some sort of real issues eg they injured you, damaged your car or made your late for work and were fired etc I would think it would be a waste of your time pursuing it.