Any idea where this water is coming from

Today after a long time I got out to my garden and saw some stagnant soapy water under decking, I have not seen this last summer, we only moved to this house in 2014, I will lift the decking and check next week, but does anybody think I need to worry and call someone before the next wekend, the black drainage pipe is from the bathroom and d the white on the left side is fom kitchen sink,
I opened the tap from both bath and kitchen and cannot see the water level increasing or moving,

Looks like teh decking has been built over a gully and the gulley is blocked. Almost certain you will need have permanent access to the gully foir cleaning it by creating a trap door in the decking.

safe to say there is a blockage at somepoint near those pipes and it will get messy but should just need unblocking and as the other person said you will need to make easy access to both gullys/drains.