Question:
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?
Answer:
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.
Sources:
Gemorah Avoda Zorah 39a, ibid Tosefos ד"ה דאמר, Y:D 118:1
