Door frames often leak as well.
Shower drain leaking down onto first floor ceiling.
Of course if the leak is severe enough that you need to put a bucket on the floor to collect the water then immediately contact your local plumber.
The most common causes are the shower pan and or a drain pipe.
One of the most common types of water leaks in a basement ceiling is due to a shower drain leaking.
How to fix a leak from the upstairs bathroom.
Use a good quality duct tape to cover the drain.
Leak from second floor shower into first floor ceiling.
The water can run outside of the shower door area onto the floor of the bathroom and leak between the shower and the floor area.
Clean the shower floor and let dry completely.
This is a very common issue with tubs.
I believe in previous post you indicated the plumber removed the shower heads capped the shower arms and turned water on.
Use this water to fill the shower until the water reaches the level of the threshold.
Packy ma if you plugged the drain and had 3 inches of standing water in the base the leak is not around the tile nor the drain nor pan.
If you have a second story bathroom the problem could be in.
I like to cover the drain and at least 6 inches on all sides.
But it also happens a lot with showers as well.
Now using a different water source such as a garden hose or another bathroom fill buckets with water.
A leaking shower is generally due to plumbers putty that has started to give away around the seals on the shower drain allowing water to seep through the cracks and onto the basement ceiling below.
No leaks were found.