"What we must do is make a three day journey into the desert. There we will be able to sacrifice to God, our Lord, just as He told us."
He then further tells Pharoah that he's going to take all the women, children and animals when he goes. Pharoah doesn't like it, but eventually (after Makat B'Chorot) relents and sends them out. But it's clear that Pharoah has been given reason to believe that we were not leaving, we were going out for a few days to serve God.
Then we see the following:
Meanwhile, the king of Egypt received the news that the people were escaping. Pharoah and his officials changed their minds regarding the people, and said, "What have we done? How could we have released Israel from doing our work?"
Now that first phrase there - he "received the news that the people were escaping." Clearly, Pharoah had not been under the impression that this was to be a permanent thing.
In other words, we left under false pretenses. At best, we allowed Pharoah to be fooled. At worst, we lied outright. Neither of these is acceptable, in my view, for the "Ohr la'goyim". The Torah tells us, "Mi'd'var sheker tirchak" - stay away from falseness. Yet we didn't exactly exemplify this here.
I haven't come up with a good answer for this yet, btw. Feel free to share your thoughts.