In my course syllabus I have office hours. However, I am happy to answer student questions if we run into each other in the hall, or if they drop by my office when I am in. However, between switching offices in buildings and a slew of committee meetings I can't say (other than my office hours) exactly when and where I can be found.
I would like a succinct phrase that conveys that my office hours are "Mondays 9-12, Tuesday 2-5 and whenever you and I happen to run into each other and it's convenient for both us to chat and you have a question".
The phrase "Mondays 9-12, Tuesday 2-5 and whenever you can find me" suggests that I am playing hide and seek.
The phrase "Mondays 9-12, Tuesday 2-5 and by mutual convenience" doesn't quite sound right to me, and not very welcoming.