Flush closet, come back in 24hr. Period & water in bowl very low.

Other then for a crack causing a leak, the only other reason is a clogged vent. In this case, water is actually sucked out from the bowl. If indeed the water drops extremely low, I would look into a crack.

Agree, usually a vent issue but it can also happen if you have a very tight home and use an exhaust fan. Do you have more than one toilet?