Clean water in shower outlet drain

Hi everyone, 
I hope you're all enjoying your vans. 
Can you please help me? 
There is a small amount of clean water sitting in the waste outlet in my shower which makes the shower floor wet when I've driven my van. 
I don't use the shower. The whole shower enclosure is dry to the touch and there aren't any visible cracks in the shower tray. I can't see or feel any water ingress or dampness from the sink, taps, shower head, toilet, or around the shower water pipe under the van. No damp patches on the ground under the shower, or anywhere else. 
There's no odour / smell and I only fill the fresh water tank to a quarter full for a wash and washing pots and make sure I don't let any food scraps go down the kitchen sink waste.
Any help or suggestions of what could be causing this and how I can resolve it would be appreciated.
Thanks in advance,
Anna 

Interesting one, try answering these, our van is identical...

Do you have a plug in the shower drains?
Don't see how the fresh tank can be an issue, we always travel with ours full.
Suspect leakage from the loo flush tank, do you use the pink stuff in it? If you do, the water in the shower would be pinkish?
Our shower head leaked until I plugged it, we never use the shower, and signs of drips from the lowest point of its hose?

Hi Ian, thanks for your reply.
There's only one drain and there's no plug.
I wasn't sure about a connection with the fresh water tank but thought I would mention it as it seems to be clean water. 
I have used the pink stuff in the past but don't use it now and the water seems to be clear but there's so little of it in the shower tray and waste it's hard to tell. The toilet flush is empty at the moment. 
How did you plug your shower head? I have used the shower in the past but not recently. 
I cant' see any signs of drips from the lowest point of the shower hose and everything else feels dry.
Thanks Anna

I’d certainly put a plug in the drain hole. That will stop any splash back from the waste tank and, if the water continues to appear, will mean it’s coming from another source such as the shower.

I removed the shower head, found an appropriate diameter piece of wood and hammered it in! So, perhaps not a fix if you want to use it!.

Fit a plug in the hole or, even just tape it over, may also help see the actual quantities involved.

My waste tank is always empty, we use a bucket under its open drain hose, so that may be worth a try too as Richard suggests

Thanks for all the replies.

Rgermain, yes I do normally travel with about 1/4 tank, I'll drain it completely before I next travel and see if that stops it.

Tinwheeler, I've bought a few plugs but can't find one to fit, it's a very shallow drain. I've even tried one of those oversized flat ones that sit on top but the water makes it move about so that's useless too. I'm sure it's not waste water as it's clear and there's no smell.

Thanks for the advice about how you plugged your shower IanH, I'll think about that one. I can try taping the drain over to see if that helps. I empty the waste tank regularly but will try the bucket under the open drain hose too.

Thanks for all your suggestions, I won't be able to try any of them until the weekend but will let you know how I get on.

Hello everyone,
I drove it this weekend with no water and found a plug that fits the hole and there has been no more water in the shower tray so hopefully that's the end of it.
Thank you all for your taking the time to reply and for the helpful advice.
Happy motor homing all.
Anna 

Well, sort of...

Water in shower tray fixed, but now no ability to travel with full water tank as you'll have to if wild camping or any other off site stops.....

Now you have the plug, I'd fill the fresh tank and take it for an energetic drive...

I drove it this weekend with no water and found a plug that fits the hole and there has been no more water in the shower tray so hopefully that's the end of it.