Instead of just a toiletries bag, just get a gym bag. Put a pair of shorts, a t-shirt, and some old tennis shoes in with your toiletries and leave it in the trunk. Even if you don't go to the gym regularly you could say you were supposed to meet a friend for tennis, jogging, or anything really, and you or they bailed. And if you leave it there all the time it will show, allowing you to say it was months ago and you just forgot it was there. Plausible deniability.
I don't have an S.O., and I still keep a gym bag with a change of clothes in my vehicle. Never know when a dancer will get glitter all over you, or a drink poured in your lap. Can't go back to work like that, can you?