What may be causing this strange leak in a bathroom sink drain?

[provlima]

Have a strange leak coming from a newly installed bathroom sink drain
and am seeking advice from experienced plumbers who may help to identify the cause.

I run the water in the sink for several minutes and there are no leals detected from the drain and its J trap connections.
When I check back 5 minutes later there are no signs of leaks.
When I check back 15 minutes later I see dampness in the grout lines below the pedestal sink.

I have checked the supply connections there are no leaks

When I check the tailpipe and J pipe connections from the drain 5 minutes after turning off the faucets, there is no evidence of water or dampness

When I check again after 15 minutes I can feel wetness

This is frustrating. Can anyone who has had a similar experience with a sink leak please chime in and help me with this frustrating problem
Thanks for your help!

 

[JCplumb]

Is water running back down the underside of the tap (faucet) and pooling where the tap meets the basin, then after time working it's way through the hole that the tap comes through?

 

[provlima]

Thanks for your reply...the water is coming from the underside of the basin drain pipe not the faucet hoses or supply valves
The mystery is there is no signs of a leak while the faucet is turned on for 5 minutes and going down the drain.
The leaking down the drain pipe doesn't start till after the faucets have been shut. The leaking looks like the bass pipe is sweating and ends up on the tiles below the sink. I cant understand this as a bad pipe connection should show the leakage while a large volume of water is flowing down the drain pipe through the J trap and into the wall
The water on the floor is coming from the drain pipe area below the basin nut

 

[Aquarius]

There could be a slight misalignment or debris where the plug hole part of the waste meets the underside of the waste. Check all around the threaded part of the waste (not the trap) first. If this is dry, then I’d be looking towards the trap. Is the water slow to go down at all?

 

[Peterdevon]

From the picture it looks like it is running down the thread, which means taking the waste out and resealing

 

[Gasmk1]

From the picture it looks like it is running down the thread, which means taking the waste out and resealing

Looks like it to me can be difficult to seal with the washer that come with the waste these can be very helpful Basin Mate | 3 Part Bathroom Basin Waste Seal Kit | Stevenson Plumbing & Electrical Supplies - https://www.stevensonplumbing.co.uk/basin-waste-seal-kit.html