* Wiring harnesses: Various electrical wires and connectors for seats, accessories, and other components.
* Possibly the passenger-side airbag module or components: This is a critical safety item, so it's possible some related parts are located under the seat.
* Loose change and debris: This is common in any vehicle!
* Floor mats: Part or all of the floor mat may be tucked under the seat.
* Small storage compartments or pockets: Some models might have small spaces built into the floor pan under the seat.
It's highly unlikely you'll find anything significantly large or unusual. To know for sure, you would need to look under the seat of that specific vehicle.