As a general rule a "bathroom" photo is not a "toilet" photo. Of course, sometimes the guidelines might allow you to just photograph any "offensive" parts of the restroom......
When I have to do a restroom photo (which, admittedly is very rare with the shops I do), I'll make a point of visiting the restroom early in my shop with the idea that I can try again later should it be too active. For instance, at a restaurant, it would not be out-of-place at all to make a visit before your meal - or even before you sit down - to wash your hands. It is certainly reasonable to make a trip during the meal. Of course, it is normal to make a stop before leaving after the meal. So, if necessary, I could make three visits to the facility and, typically would have an empty restroom at least once.
I'm not keen on bathroom photos when other folks are making use of the facility. - it's just not.... Anyhow, I have seen strange phone usage in men's rooms and suppose I could still take a clandestine picture if I had to. I'll never forget the guy who made the mistake of texting at the urinal - and dropping his phone. (I did not stick around long enough to find out if he decided to try and salvage it.)
Hard work builds character and homework is good for your soul.