HELP! Our toilet is at least 15 yrs old - maybe much older. Several months ago, it developed a slow leak around the base. My husband replaced the seal (twice) but it continues to have a slow, small leak. No signs of a crack or other issues... a small towel around the base has temporarily worked. Tonight, we noticed there was very little water in the bowl even though it wasn't flushed. When we flush it, the bowl quickly fills up to the seat level with water. Once the toilet stops running/flushing, the water slowly drains out so that the water level is VERY low. It's the only toilet we have! HELP!

Sounds to me that you have a hidden crack in the bowl. This could be caused by something dropped in it or even a manufactures defect from years ago.
Sorry but this calls for a replacement. Pull the old toilet and install a new one. Good luck. Tom