re: strange?

If he has no phone, no facebook.. is always with you etc..
I don't see HOW he could be cheating.

And if he really wanted to hide it, wouldn't it have gone under his chair or something?
At first it sounded shaded but I think his reasoning could be legit.

I've picked up stuff that wasn't mine before if I've been hanging around friends etc.

You're a SAHM. Can you recall a time where he's left early or came home late? It doesn't sound like he's cheating if his car is generally messy, he prob thought it was your shirt and fell out of the car. And the whole jacket thing, if he normally throws it there, that's prob just what happened. Don't over analyze it unless it seems more fishy from this point out. Just keep eye, that's all.