Water sloshing around at base of windshield

My 01 Montana EWB was always garaged and never exhibited signs of water leakage, until keeping it outside for the past several months. Sometimes, but not always, after it has rained, moisture is visible inside the bottom of the windshield where the VIN stamp is. I check it frequently. Just the other day I drove it and heard water sloshing back-and-forth underneath the dash panel at the base of the windshield when turning and/or accelerating. There is no rust along the top of the windshield and I've seen no evidence of water leaking anywhere inside, either from the headliner or overhead console. Where could this water be coming in from if it's only at the base of the windshield, with no evidence of leakage higher up? My driveway is quite flat and level. Also the van is seldom driven, so the leak happens while it sits. Thanks!