Quite often at work I hear Thai people use the word "mung" at the end of sentences. It is used when they are saying something like "maybe". But I thought "aht ja" and "bang tee" means "maybe". What are the rules for "mung"?

The word "mung" มั้ง means "perhaps" or "I guess". You would use it at the end of sentences like this:

ter bai laew mung (she has already gone I guess)
เธอ ไป แล้ว มั้ง

tam-mai Lek yung mai ma? (Why hasn't Lek come yet?)
ทำ ไม เล็ก ยัง ไม่ มา

rot dtit mung (Maybe there is a traffic jam)
รถ ติด มั้ง

It is something I would say in conversation to my friends but I wouldn't usually say it to my teacher

to teacher: aht ja bai laew krap (probably already gone)
to friends: bai laew mung (I guess he has already gone)