inventory_obs_field/inventory_obs_field_view.xml

<?xml version="1.0"?>
<openerp>
    <data>

        <record id="view_inventory_obs_form" model="ir.ui.view">
            <field name="name">inventory.obs.field.inherit</field>
            <field name="model">stock.inventory</field>
            <field name="inherit_id" ref="stock.view_inventory_form"/>
            <field name="arch" type="xml">

                    <field name="location_id" position="after">
                        <field string="Motivo:" name="obs_inventory" placeholder="Motivo del Inventario" />
                    </field>

            </field>
        </record>
    </data>
</openerp>