We have water under our kitchen sink but no obvious leaks. ( we have had a plumber out but he could not identify issue. The water occurs after using upstairs shower which is not above the kitchen. We were told it could not be connected but that is the only time we notice it. We have tested dishwasher, garbage disposal , sink. Etc.
Any thoughts for how to proceed?

take a 5 gallon bucket full of water and carefully dump it down the shower drain.
look for the water to show up under the sink.
if it doesn't show up then the shower drain and drain pipe are not the source.
the next thing to isolate is the shower head piping. you do that by removing the head and capping the pipe. turn the water on for a few minutes and if that is the source , you'll know it.
next step is to check the walls of the shower which you can do by taking a shower.
so what i'm saying is you break it down into three separate tests.
1) shower drain
2) shower piping
3) shower walls

"water under our kitchen sink" can you clarify what this means?

You mean water is seeping out from under the toe kick onto the kitchen floor?

or water is seen inside the cabinet floor under the kitchen sink that forms a puddle inside?

In addition to Packy's excellent suggestions, you might also try spreading toilet paper or tissues across the floor of the cabinet. The light weight tissues can show you where the drops or spray is coming from when they get hit by a tiny bit of water.