Thank you for giving me a heter, as I was concerned that I had done an aveira by eating the pizza.
However, please see here http://www.shemayisrael.com/parsha/halacha/Issue6.pdf that according to Rav Belsky zatzal, it would seem preferable for the pizza to be sealed.
With all due respect, to say that the professor is concerned that I will go to the store and make inquiries is a bit of a stretch. There are dozens of people going through the store, so how would they remember anyway?
Your case was bedieved (postfacto) after the pizza was bought and one slice eaten, but you are correct that it is preferable for the pizza to be sealed.
Gemorah Avoda Zorah 39a, ibid Tosefos ד”ה דאמר, Y:D 118:1