AssalaamuAlaikum WRWB,
Brother,
I think the kind of action is not will not be considered good. Sorry for this harsh statement, but even nonMuslims have developed a sense about how evil is telling lies.
While you were in a nonMuslim environment, it was your duty to make people notice that "you are DIFFERENT" , you should had been a source of inspiration for your fellows that you are a Muslim.
Secondly, its not essential to tell the customers that you are in restroom or offering prayer. Just simply pass on the message that you are "Busy for 5 minutes". Thats it. I think you can at least offer Fardh prayer in 5 to 10 minutes maximum. This way, you wont be telling them lies.
After all, it is a kind of shameful thing that you tell customers that you stay half of your time in restrooms.
I wonder, if people were telling their fellows about washrooms, then how did Islam spread? How nonMuslims got inspiration from Muslims?
The first percepition of many nonMuslims in the history was that:
1- They observed a Muslim, then they thought he is odd/different.
2- They felt awkward, why does he pray 5 times, or why does he do certain things the certain way.
3- This strange perception about that Muslim caused them to be curious to know what is actually that faith he has, that he leaves anything just to praise his God, what kid of God is that this Muslim person is so much dedicated about?
4- This curiosity or strange feeling motivates the people around that Muslim to study about Islam. (what is Islam OR what the heck is that religion which makes this guy so much dedicated).
5- They study Islam.
6- Many of them find truth.
Why the heck will anybody get inspiration from the character of Muslims if Muslims themselves are "NO DIFFERENT"???
Wassalaam