One of two things: It may be slighty clogged which can cause a siphon effect. Try vigorously plunging to clear. Given your question its unlikely you have a snake, but if you do, run it down the toilet to clear the clog.
More serious problem might be a cracked colon in the toilet base(no kidding...its called a colon) which is allowing a leak down. I don;t have a cross section picture of a toilet, but essentially the flow route thru the base looks somewhat akin to a P-trap ( the U shapped pipe under your sink) that allows water to stay in the bowl when the tank refills so as to not allow sewer gases to enter from the drain lines. If this is the case ya gotta replace your toilet.
Fortunately, it is more likely a partial clog,,,,
And yes, I did stay at a Holiday Inn Express last nite
