You are only matching on warehouse. So your query basically says if ANY row in that warehouse has the quantity you are looking for, return ALL the rows in that warehouse.
To get what you want, you would have to match on warehouse, product, and model, e.g.,
Select warehouse, product, model, quantity
From Inventory I1
Where 50 In
(Select quantity From Inventory I2
Where I1.warehouse = I2.warehouse
And I1.product = I2.product
And I1.model = I2.model)
Tom