It'll be water ingress into the unlocking button on the tailgate itself, I replaced 1 last week for exactly the same fault. Could try drying it out, but would probably be best replacing the switch.
Just to double confirm this symptom - My 2012 (61 plate) Clio estate had the tailgate replaced due to a rear shunt last September and since then I have had various problems relating to the central locking and the boot lid latch (most often during or after wet weather). Sometimes it would pop open when the car was unlocked and at other times it would stay locked when the car was unlocked. A handful of times the tailgate unlatched when driving.
When the tailgate was open, water would often run out of the latch mechanism at the bottom of the tailgate (usually down my back).
There was rain water entering in three areas.
1) The fixings for the "spoiler" at the top of the tailgate.
2) The Renault Diamond badge fixing holes.
3) The black plastic strip which houses the number plate lights and the boot (un)latch button .
When the tailgate had been replaced the seals in all these areas had been damaged. I cleaned, dried and sealed all of them with some silicone sealant. The tailgate release switch has a rubberised seal where the housing joins but it is not very good. I split the switch housing, dried it properly (it was very wet) and sealed it with a line of silicone sealant.
All good now and not a difficult job (Only tools required are a torx driver for the panel and 10mm deep socket for the spoiler). I was concerned that it might be caused by trapped wiring or a faulty BCM (body control module) but strongly suspected the repair since the problem did not exist before then.