Probably not a leak...

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Sump pumps use a float switch (much like a toilet) to turn the pump on and off. In addition, the discharge pipe leading away from the pump to the outside has a backflow valve in it. A problem with either could cause the problem you are having.

To detect a bad float switch, slowly fill the sump area with water (on a dry day). Observe when the pump comes on and off. When the water rises to a cetain level, the pump should cycle on. As the water-level sinks, the pump should turn off. If it does not turn or of at the proper time, then just buy a new pump ($50 or so).

Also check to make sure that the float is unobstructed and free to move up and down. Sometimes the cord can wrap around the float and keep it from moving up and down.

If the pump cycles on and off continously, then you could have a stuck backflow valve, or a clogged or bad pump. The backflow valve is a oneway valve that allows the water to be ejected outside, but then prevents the water remaining in the pipe from flowing back down into the sump area (the ejection pipe usually rises some distance to get the water above grade). If this valve is gummed up, it may stick open and allow the water to seep back into the sump area. ***This is very unlikely***

Bottom line is that the answer is usually a bad pump, or a bad switch. In either case, the answer is to change the pump. This is a very simple repair...just unscrew the ejection pipe, and unplug the pump...then do this in reverse with the new pump. It will cost you $50 dollars and 1 hour. Well worth the piece of mind!