This is gross, but unfortunately people do cheat in their cars. Think of a car as a mobile bedroom. Cheaters will often use their cars as convenient places to have sex. Unfortunately cars also retain odors. If you start to smell a constant musty smell, something fishy might be going on. Also beware if their car always has air fresheners. They may be trying to mask the smell of something.
Has your lover started performing new sexual moves? Are they trying to get you to try things you have never done before? Are they suddenly interested in new sexual activities that they were previously uninterested by? Your cheating lover could be learning these new tricks from the person that they are cheating with. Be aware if you notice your lover suddenly having new fetishes for apparently no reason.
Has the chemistry of your relationship suddenly changed? There’s a familiarity that comes with regular sex with your lover. Adding a third person to this mix can affect the chemistry of the sex in your relationship. Are you bumping heads when you lean into kiss? Are you finding that your partner just isn’t getting aroused by the same things anymore? This is more of an intuition thing, but if you feel that for some reason things are just “off” in your sexual relations, there may be something else going on.