Tire went completely flat randomly overnight then kept air

So I'm pretty new to cycling, I have an entry level mountain bike that's about 2 months old. I road it on the trails for a few hours on Saturday without any noticeable issues. Parked it inside afterwards and woke up to an almost completely deflated tire. If I squeezed it I could hear air leaking, wasn't sure where.

I pumped it back up to 40 PSI, didn't hear any leakage even when squeezing. Let it sit overnight and almost all of today and the tire still has air and doesn't look to have leaked at all. I'm not really sure if I should take the tube out and try to find the leak (not something I've done before) or let it go and see what happens. Is this a normal occurrence?